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Materials such as wood or metal which are at equal temperatures are perceived to be of different ldquocoldnessrdquo due to differences in thermal properties, such as the thermal diffusivity. The thermal diffusivity of a material is a parameter that controls the rate with which heat is extracted from the hand when it touches an object of that material. This rate of heat extraction is an important cue for distinguishing materials and recognising objects by means of touch. We have measured the ability of human observers to discriminate between different rates of heat extraction. This was done using a device that displayed different transient temperature profiles to the finger. In different conditions, subjects were repeatedly asked to select the faster-cooling of two stimuli. The discrimination threshold was around 43% of the extraction rate. A rate that was twice as slow also yielded twice the absolute discrimination threshold. When we halved the temperature difference between beginning and end of the stimulus, the threshold did not change as much. This shows that subjects can use the rate of heat extraction as a cue and that they can discriminate between materials if their thermal diffusivities are at least 43% apart.