Weight and linear dimensional changes of two hydrocolloid duplicating materials following setting in various environments were examined. Prior to the measurements, the sample materials were stored either in the sol or in the gel phase, for a period of 5 days, in order to evaluate the effect of ageing on their composition and behaviour. A 100% relative humidity environment produced the least change to the set material, and in this environment, the behaviour of the materials was not affected by their age. Results indicate that deterioration of the duplicating materials provoked by repeated or prolonged heating, is detectable, through their weight changes, even at an early stage.