Leather belts were the first to be used in drive technology. Over time, they were gradually replaced by belts made of synthetic materials. In recent years, there has been a resurgence in the use of such materials. However, these are now belts constructed as a sandwich structure  with an outer leather layer and an inner layer made of polyamide. This article presents an analysis of the thermal condition of a leather-belt transmission system depending on the braking torque load at various rotational speeds of the driving pulley. Surface roughness measurements enabled the evaluation of belt wear. The surface morphology of the belts was determined using scanning electron microscopy (SEM) and optical microscopy. The research results can be applied to assess belt wear, their thermal stability, and the overall reliability of belt transmissions.