Results of laboratory determinations of thermal conductivities in the temperature range −150 to 540 deg C are presented for 12 iron-nickel alloys. Six samples are of low nickel content, in the range from 1 to 9 per cent, and six others have nickel contents in the range from 35 to 80 per cent. A sample of AISI 1015 steel is included for comparative purposes. The determinations were made on bar specimens about 2.54 cm in diameter and 37 cm long, by an absolute steady-state method with heat flowing longitudinally in the bar. Computation of results from observed data was effected by means of a digital computer.