A method and device for ascertaining at least one variable regarding a state of a brake fluid in a brake system of a vehicle includes a device determining a volume flow variable regarding a volume flow, which flows via a simulator separation valve into a simulator by taking into account at least one displacement speed variable regarding a displacement speed of a brake pedal/rod piston; determining a simulator internal pressure variable regarding a simulator internal pressure by taking into account at least one pedal travel variable regarding a pedal travel; determining a pressure difference variable regarding a pressure difference at the simulator separation valve by taking into account the simulator internal pressure variable and an admission pressure variable regarding an admission pressure; and determining at least one variable by taking into account the at least one determined volume flow variable and the at least one determined pressure difference variable.