Microphones can be calibrated by a primary technique in a diffuse sound field. The formula for the diffuse‐field voltage response is derived from the well‐known relationship for a reciprocal transducer in free space. It is shown that the reciprocity parameter for a diffuse‐sound field follows from that for free space by replacing the distance by the “diffuse‐field distance” of a point source. This distance depends only on the total absorption in the reverberation room. The experimental procedure is described in some detail and the results of the diffuse‐field calibration of a Western Electric 640 AA condenser microphone are presented.