We report on measurements of the spontaneous emission factor for oxide-confined InGaAs vertical cavity surface emitting lasers. The spontaneous emission factor is determined as a function of the active layer volume from the measurement of small-signal harmonic distortion at threshold. For a 3×3 &mgr;m oxide aperture device we obtain spontaneous emission factor of4.2⋅10−2at room temperature. ©1997 American Institute of Physics.