The technique whereby the reciprocity theorem is applied to the determination of the pressure sensitivities of microphones has been modified to improve the reliability and accuracy of the measurements. Sensitivities are measured in terms of the volume of a cavity, the capacity of a fixed condenser, and the variable setting of an accurate potentiometer. Six different pairs of measurements are made instead of three, which is the minimum possible number: this enables check to be made on the internal consistency of each calibration. The accuracy of the measured sensitivities is estimated to be ±0.05 db on an absolute scale and ±0.03 db relative to each other. This technique has been employed to measure the properties of several condenser microphones of each of three well known types over periods ranging from five months to two years. Results are given for their temperature and pressure coefficients, effective volumes due to nonrigidity of their diaphragms and drift with the passing of time.