Pump–probe measurements are presented of gain compression and recovery in a series of four semiconductor optical amplifiers of different lengths but otherwise identical structures. A continuous wave probe beam from a tunable laser is used to measure the wavelength dependence of gain compression and recovery times. A good level of agreement is obtained when these results are compared with numerical simulations using a model that includes the material gain spectrum, saturation effects and the variation of optical intensities with longitudinal position in the device. In addition the experimental results are interpreted in terms of a recent theory that offers approximate analytic expressions connecting recovery time with transmission gain. The spectral dependence of the recovery time and the gain are used to verify the correlations between these quantities and their dependence on device length. The results for recovery time show a strong dependence on probe wavelength with a pronounced minimum which is coincident with the peak of the gain spectrum for each device. A rather weak correlation of speed with length is found.