c.w. X-band Gunn oscillators made with n + cathode contacts or carefully controlled metal-alloy contacts have good frequency stability against changes of ambient temperature when operated in the appropriate conditions. It is shown that the high stability behaviour is controlled by the ratio of the cavity-controlled frequency to the transit frequency. The optimum ratio is close to that which gives the optimum efficiency. A physical explanation is derived from a finite-difference computer simulation of the depth of r.f. current modulation and its phase relative to the controlling r.f. voltage.