A transient technique for the measurement of the thermal conductivity of non-metals is described. A small sample of material is held between a hot copper block and a cold base, and the thermal conductivity of the material is deduced from the exponential decay in the temperature difference between the copper block and the base. The influences of heat losses from the block and contact resistance are established by calibration. Very rapid measurements may be made to an accuracy of about ±5% over the thermal conductivity range 0.1–1 W/m K, although thermal conductivities outside this range may also be measured with careful operation using samples of an appropriate size. The accuracy and repeatability of the results are demonstrated by the results of tests on small resin samples.