Abstract 1. We measured and simulated thermal gradients within operative temperature models of reptiles. 2. In thin-walled models of moderate to large sized organisms, thermal gradients , induced largely by solar radiation, can be large (>10°C). 3. Thicker model walls attenuate the regional variation in temperature. Sufficiently thick walls can keep regional temperature differences smaller than 1°C under all simulated conditions. 4. Thicker walls also increase the thermal inertia of models, but time constants of the models will always be faster than those of the real animals and allow the model to sample the range of temperatures available to the animal. 5. By reducing spatial temperature gradients within models, thick walls can improve operative temperature estimates for moderate to large-sized reptiles.