The thermal relaxation time constant of several Allen Bradley‐type resistors used in low‐temperature thermometry are reported. The results indicate that thermal diffusion within the carbon composite controls this characteristic time and that it is independent of surface heat transfer. A simple formulation based on published data shows that the proper functional form and magnitude are predictable. It is most probable that these results can be used to estimate the time constant for resistors of other dimensions. The results confirm that the most effective method for decreasing the thermal time constant of carbon composition resistors is by reducing their thickness.