diff --git a/src/pybamm/models/submodels/electrolyte_diffusion/constant_concentration.py b/src/pybamm/models/submodels/electrolyte_diffusion/constant_concentration.py
index 006619e8bb..db05a2902c 100644
--- a/src/pybamm/models/submodels/electrolyte_diffusion/constant_concentration.py
+++ b/src/pybamm/models/submodels/electrolyte_diffusion/constant_concentration.py
@@ -1,10 +1,11 @@
-#
-# Class for leading-order electrolyte diffusion employing stefan-maxwell
-#
 import pybamm
+from typing import Optional, Any, TYPE_CHECKING
 
 from .base_electrolyte_diffusion import BaseElectrolyteDiffusion
 
+if TYPE_CHECKING:
+    from pybamm import ParameterValues
+
 
 class ConstantConcentration(BaseElectrolyteDiffusion):
     """Class for constant concentration of electrolyte
@@ -17,19 +18,21 @@ class ConstantConcentration(BaseElectrolyteDiffusion):
         A dictionary of options to be passed to the model.
     """
 
-    def __init__(self, param, options=None):
+    def __init__(
+        self, param: "ParameterValues", options: Optional[dict[str, Any]] = None
+    ) -> None:
         super().__init__(param, options)
 
-    def get_fundamental_variables(self):
-        c_e_init = self.param.c_e_init
-        eps_c_e_dict = {
+    def get_fundamental_variables(self) -> dict[str, pybamm.Symbol]:
+        c_e_init: float = self.param.c_e_init
+        eps_c_e_dict: dict[str, pybamm.Symbol] = {
             domain: self.param.domain_params[domain.split()[0]].epsilon_init * c_e_init
             for domain in self.options.whole_cell_domains
         }
-        variables = self._get_standard_porosity_times_concentration_variables(
-            eps_c_e_dict
+        variables: dict[str, pybamm.Symbol] = (
+            self._get_standard_porosity_times_concentration_variables(eps_c_e_dict)
         )
-        N_e = pybamm.FullBroadcastToEdges(
+        N_e: pybamm.Symbol = pybamm.FullBroadcastToEdges(
             0,
             [domain for domain in self.options.whole_cell_domains],
             "current collector",
@@ -39,13 +42,17 @@ def get_fundamental_variables(self):
 
         return variables
 
-    def get_coupled_variables(self, variables):
-        c_e_dict = {}
+    def get_coupled_variables(
+        self, variables: dict[str, pybamm.Symbol]
+    ) -> dict[str, pybamm.Symbol]:
+        c_e_dict: dict[str, pybamm.Symbol] = {}
         for domain in self.options.whole_cell_domains:
             Domain = domain.capitalize()
-            eps_k = variables[f"{Domain} porosity"]
-            eps_c_e_k = variables[f"{Domain} porosity times concentration [mol.m-3]"]
-            c_e_k = eps_c_e_k / eps_k
+            eps_k: pybamm.Symbol = variables[f"{Domain} porosity"]
+            eps_c_e_k: pybamm.Symbol = variables[
+                f"{Domain} porosity times concentration [mol.m-3]"
+            ]
+            c_e_k: pybamm.Symbol = eps_c_e_k / eps_k
             c_e_dict[domain] = c_e_k
 
         variables["Electrolyte concentration concatenation [mol.m-3]"] = (
@@ -53,21 +60,21 @@ def get_coupled_variables(self, variables):
         )
         variables.update(self._get_standard_domain_concentration_variables(c_e_dict))
 
-        c_e = (
+        c_e: pybamm.Symbol = (
             variables["Porosity times concentration [mol.m-3]"] / variables["Porosity"]
         )
         variables.update(self._get_standard_whole_cell_concentration_variables(c_e))
 
         return variables
 
-    def set_boundary_conditions(self, variables):
+    def set_boundary_conditions(self, variables: dict[str, pybamm.Symbol]) -> None:
         """
         We provide boundary conditions even though the concentration is constant
         so that the gradient of the concentration has the correct shape after
         discretisation.
         """
 
-        c_e = variables["Electrolyte concentration [mol.m-3]"]
+        c_e: pybamm.Symbol = variables["Electrolyte concentration [mol.m-3]"]
 
         self.boundary_conditions = {
             c_e: {
@@ -76,6 +83,6 @@ def set_boundary_conditions(self, variables):
             }
         }
 
-    def add_events_from(self, variables):
+    def add_events_from(self, variables: dict[str, pybamm.Symbol]) -> None:
         # No event since the concentration is constant
         pass