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A design of a four square gear test rig that allows the wear of polymer and composite gears to be monitored continuously during operation is described. The wear behavior of three typical gear materials is examined and it is shown that the wear characteristics differ greatly. For Acetal there is a sharp rise in wear as the transmitted torque is increased, effectively limiting the torque that can be transmitted by an Acetal gear pair. This wear transition is shown to be associated with the maximum surface temperature of the gear reaching the melting point of Acetal. This provides an indirect method of checking the accuracy of existing models for the maximum surface temperatures in polymer gears. Very large discrepancies are found. A simple, alternative model for the average and maximum gear temperatrues is developed and is shown to predict the wear transition with far greater accuracy.