This study uses simultaneous observations of storm time Pc 5 events by the GOES 2 and GOES 3 geostationary satellites to study the correlation of Pc 5 waves with substorm onsets. Eighteen Pc 5 events occurring from March to December 1979 are first surveyed. After excluding events with highly varying activity or scarce station coverage, only six events with clear substorm features 2--4 hours before the GOES observation of the Pc 5 events are then analyzed. From the wave propagation speed and the distance from local midnight to the satellite position, the substorm onset time is estimated. For the six events examined, ground magnetograms near local midnight indicate that a substorm onset occurred within 20 min of the estimated substorm onset times. This result suggest that the occurrence of storm time Pc 5 waves is probably correlated with substorm onsets. ¿ American Geophysical Union 1990