Measurements of the nonlinear refractive index of a GaInAs/InP multiple quantum well structure are reported. Optical absorption data obtained by a pump and probe technique were analyzed using the Kramers–Kro¨nig relations to give index change as a function of wavelength for various pump intensities. The results indicate that optical devices should be realizable in this material system. The importance of saturation of the nonlinearity is demonstrated and the behavior shown to be well described by a simple band filling model.