A unique method has been developed to completely determine the wavevector of a mirror mode wave using data from two closely spaced (separation less than wavelength/2) spacecraft. The method is based on the fact that mirror mode waves are linearly polarized and have zero frequency in the plasma rest frame. The method has been tested using Active Magnetospheric Particle Tracer Explorers United Kingdom Subsatellite (AMPTE UKS) and Ion Release Module (AMPTE IRM) high-resolution magnetic field data from an interval of mirror mode waves observed in the middle magnetosheath on September 21, 1984. The results are also compared to those obtained using other methods of wavevector determination. The wavevectors estimated during the event are highly oblique to the ambient magnetic field direction as is expected from mirror wave theory. The application of the wavevector determination technique to further studies, such as studying the effect of finite electron temperature on mirror wave characteristics, is discussed. ¿ 1999 American Geophysical Union