diff --git a/vizier/_src/benchmarks/experimenters/normalizing_experimenter_test.py b/vizier/_src/benchmarks/experimenters/normalizing_experimenter_test.py
index 66ae2e45a..4d8ab7457 100644
--- a/vizier/_src/benchmarks/experimenters/normalizing_experimenter_test.py
+++ b/vizier/_src/benchmarks/experimenters/normalizing_experimenter_test.py
@@ -75,7 +75,7 @@ def testNormalizationApply(self, func):
 
   def test_NormalizingCategoricals(self):
     mab_exptr = multiarm.FixedMultiArmExperimenter(
-        rewards=[-1e6, 0.0, 1e6], arms_as_chars=False
+        arms_to_rewards={'0': -1e6, '1': 0.0, '2': 1e6}
     )
     norm_exptr = normalizing_experimenter.NormalizingExperimenter(mab_exptr)
     metric_name = norm_exptr.problem_statement().metric_information.item().name
diff --git a/vizier/_src/benchmarks/experimenters/synthetic/multiarm.py b/vizier/_src/benchmarks/experimenters/synthetic/multiarm.py
index 88ea5d825..6a015de42 100644
--- a/vizier/_src/benchmarks/experimenters/synthetic/multiarm.py
+++ b/vizier/_src/benchmarks/experimenters/synthetic/multiarm.py
@@ -20,32 +20,20 @@
 distributions.
 """
 
-import copy
-from typing import Optional, Sequence
+from typing import Mapping, Optional, Sequence
 
 import numpy as np
 from vizier import pyvizier as vz
 from vizier._src.benchmarks.experimenters import experimenter
 
 
-def _default_multiarm_problem(
-    num_arms: int, arms_as_chars: bool
-) -> vz.ProblemStatement:
+def _default_multiarm_problem(arms: Sequence[str]) -> vz.ProblemStatement:
   """Returns default multi-arm problem statement."""
   problem = vz.ProblemStatement()
   problem.metric_information.append(
       vz.MetricInformation(name="reward", goal=vz.ObjectiveMetricGoal.MAXIMIZE)
   )
-
-  if arms_as_chars:
-    # Starts with 'a' character.
-    feasible_values = [chr(i + 97) for i in range(num_arms)]
-  else:
-    feasible_values = [str(i) for i in range(num_arms)]
-
-  problem.search_space.root.add_categorical_param(
-      name="arm", feasible_values=feasible_values
-  )
+  problem.search_space.root.add_categorical_param("arm", feasible_values=arms)
   return problem
 
 
@@ -54,23 +42,24 @@ class BernoulliMultiArmExperimenter(experimenter.Experimenter):
 
   def __init__(
       self,
-      probs: Sequence[float],
-      arms_as_chars: bool = True,
+      arms_to_probs: Mapping[str, float],
       seed: Optional[int] = None,
   ):
-    self._probs = probs
+    if sum(arms_to_probs.values()) != 1.0:
+      raise ValueError(
+          "Sum of probabilities must be 1, got %s" % sum(arms_to_probs.values())
+      )
+    self._arms_to_probs = arms_to_probs
     self._rng = np.random.RandomState(seed)
-    self._problem = _default_multiarm_problem(len(self._probs), arms_as_chars)
 
   def problem_statement(self) -> vz.ProblemStatement:
-    return copy.deepcopy(self._problem)
+    return _default_multiarm_problem(list(self._arms_to_probs.keys()))
 
   def evaluate(self, suggestions: Sequence[vz.Trial]) -> None:
     """Each arm has a fixed probability of outputting 0 or 1 reward."""
-    feasibles = self._problem.search_space.parameters[0].feasible_values
     for suggestion in suggestions:
-      arm_index = feasibles.index(suggestion.parameters["arm"].value)
-      prob = self._probs[arm_index]
+      arm = suggestion.parameters["arm"].value
+      prob = self._arms_to_probs[arm]
       reward = self._rng.choice([0, 1], p=[1 - prob, prob])
       suggestion.final_measurement = vz.Measurement(metrics={"reward": reward})
 
@@ -78,16 +67,14 @@ def evaluate(self, suggestions: Sequence[vz.Trial]) -> None:
 class FixedMultiArmExperimenter(experimenter.Experimenter):
   """Rewards are deterministic."""
 
-  def __init__(self, rewards: Sequence[float], arms_as_chars: bool = True):
-    self._rewards = rewards
-    self._problem = _default_multiarm_problem(len(self._rewards), arms_as_chars)
+  def __init__(self, arms_to_rewards: Mapping[str, float]):
+    self._arms_to_rewards = arms_to_rewards
 
   def problem_statement(self) -> vz.ProblemStatement:
-    return copy.deepcopy(self._problem)
+    return _default_multiarm_problem(list(self._arms_to_rewards.keys()))
 
   def evaluate(self, suggestions: Sequence[vz.Trial]) -> None:
-    feasibles = self._problem.search_space.parameters[0].feasible_values
     for suggestion in suggestions:
-      arm_index = feasibles.index(suggestion.parameters["arm"].value)
-      reward = self._rewards[arm_index]
+      arm = suggestion.parameters["arm"].value
+      reward = self._arms_to_rewards[arm]
       suggestion.final_measurement = vz.Measurement(metrics={"reward": reward})