Current stress relaxation measurements in compression and the associated equipment are not reliable enough to carry out prolonged tests, especially at high temperatures and in liquid environments. The methods of measurement are outlined with the greatest flexibility in BS 903 Part 34, yet the consideration given to overcome the difficulties encountered is limited. The objectives of this sponsored programme were to examine the best available equipment, to generate a set of accurate data including some at relatively high temperatures; and to formulate a technique to extrapolate these high temperature results to service temperatures.