The heat loss from wires in nitrogen, helium, and mixtures thereof has been measured at low Reynolds numbers and has been compared with the theory of Kassoy. The data in nitrogen agree well with previous data and with the theory for continuum flow and validate the experimental techniques. The helium data exhibit large slip effects at low Knudsen numbers due to the low thermal accommodation coefficient of helium; good agreement with the theory of Kassoy for slip effects is found. Some data exhibiting slip in helium‐nitrogen mixtures are given. A mixture rule for the effective thermal accommodation coefficient in binary mixtures is suggested.