In-service monitoring of the customer's phosphate ester turbine oils has been practiced for over 15 years. This service originated with users of electrohydraulic control (EHC) fluids and gradually expanded to include specialty-type synthetic triaryl phosphate ester lubricants for large and medium-sized gas turbines and turbine-compressor operations. In-service monitoring is intended to assist the user in caring for the fluid to obtain maximum service life. Users who avail themselves of this service have their used fluids examined for various properties, which could include as many as seven measurements: viscosity, acid number, specific gravity, water content, mineral oil content, fire properties, and stability. Corrective actions are suggested if a fluid exceeds certain property measurements.