The field‐dependent energy relaxation times of electrons in GaAs and InP have been determined from the 34‐GHz complex conductivity measured in the presence of a dc heating field. The values for GaAs and InP are 2.5 and 1.7 psec, respectively, at low fields, but increase rapidly near the bulk‐effect threshold field values. Moreover, the relaxation time for InP is found to be less than that for GaAs at all fields. These results are in qualitative accord with predictions based on velocity‐field measurements.