The test method describes the procedure for determination of the total acid number of new and degraded polyol ester and diester based gas turbine lubricants by potentiometric titration technique. The method was validated to cover an acidity range 0.05 to 6.0 mg KOH g\u-<(hoch)1>. The method may also be suitable for the determination of acidities outside of this range and for other classes of lubricant.