diff --git a/ratapi/inputs.py b/ratapi/inputs.py
index a537b21..921890b 100644
--- a/ratapi/inputs.py
+++ b/ratapi/inputs.py
@@ -385,7 +385,7 @@ def make_problem(project: ratapi.Project) -> ProblemDefinition:
 
 def get_layer_details(project: ratapi.Project) -> list[int]:
     """Get parameter indices for all layers defined in the project."""
-    hydrate_id = {"bulk in": 1, "bulk out": 2}
+    hydrate_id = {"bulk in": 0, "bulk out": 1}
     layer_details = []
 
     # Get the thickness, SLD, roughness fields from the appropriate model
diff --git a/tests/test_inputs.py b/tests/test_inputs.py
index bf9a2e1..244fd4d 100644
--- a/tests/test_inputs.py
+++ b/tests/test_inputs.py
@@ -175,7 +175,7 @@ def standard_layers_problem(test_names, test_checks):
     problem.numberOfContrasts = 1
     problem.numberOfLayers = 1
     problem.repeatLayers = [1]
-    problem.layersDetails = [[2, 3, 4, float("NaN"), 2]]
+    problem.layersDetails = [[2, 3, 4, float("NaN"), 1]]
     problem.contrastLayers = [[1]]
     problem.numberOfDomainContrasts = 0
     problem.domainContrastLayers = []
@@ -243,7 +243,7 @@ def domains_problem(test_names, test_checks):
     problem.numberOfContrasts = 1
     problem.numberOfLayers = 1
     problem.repeatLayers = [1]
-    problem.layersDetails = [[2, 3, 4, float("NaN"), 2]]
+    problem.layersDetails = [[2, 3, 4, float("NaN"), 1]]
     problem.contrastLayers = [[2, 1]]
     problem.numberOfDomainContrasts = 2
     problem.domainContrastLayers = [[1], [1]]