We describe measurements of dynamic wavelength shift in single mode vertical cavity surface emitting lasers operating at 850 nm. Small signal modulation experiments yield a linewidth enhancement factor 3.7<&agr;<4.5, depending on the modulation frequency. Under large signal modulation we measure &Dgr;&ngr;×&Dgr;&tgr;∼3.6 for gain switched pulses. The dependence of dynamic chirp on the pulse width is accurately modeled by numerical solution of rate equations. ©1995 American Institute of Physics.