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Summary form only given. It is well known that the performance of vertical-cavity surface-emitting lasers (VCSELs) is limited by saturation and eventual roll-over of their output power. This is due to the device heating produced by the injection current, J/sub inj/ which produces a movement of the cavity mode across the gain spectrum and an increase in thermally activated, leakage currents. Here we present a method to measure the optical gain from a VCSEL structure and use the results to analyse the thermal roll-over effects within the devices. The measurement technique uses a multi-section, edge-emitting configuration which allows the determination of the optical gain for different device lengths. As this is a single pass transmission experiment there is no limitation of the gain to a threshold value nor a requirement for an in-plane waveguide.