We have investigated the effect of mismatch between the reflectivity resonance and the gain spectra of vertical cavity surface emitting lasers. Detuning was caused by hydrostatic pressure and self‐heating. Hydrostatic pressure shifts the gain peak towards shorter wavelengths with respect to the Fabry‐Pe´rot (FP) resonance without modifying the gain spectrum. The threshold current remained unchanged for a positive mismatch of up to 18 nm. It increased four‐fold for a negative mismatch of −13 nm at 0.5 GPa, where lasing disappeared. Increased threshold current and quenching of the emission are a consequence of the decrease of the gain at the FP resonance. A similar effect was observed when the gain peak was red‐shifted with respect to the FP resonance by increasing the injected power. Increased mismatch is accompanied by a decrease of the gain at the emission wavelength which is responsible for the laser output quenching at high injected powers. ©1995 American Institute of Physics.