This part of ISO 3384 specifies two methods for determining the decrease in counterforce exerted by a test piece of vulcanized or thermoplastic rubber which has been compressed to a constant deformation and maintained thus at a predetermined test temperature. The counterforce can be determined either by means of a continuous-measurement system or by a discontinuous-measurement one. Two forms of test piece are permitted: cylindrical test pieces and rings. Different shapes and sizes of test piece give different results, and comparison of results should be limited to test pieces of similar size and shape. The use of ring test pieces is particularly suitable for the determination of stress relaxation in liquid environments. This part of ISO 3384 deals only with testing at constant ambiant or elevated temperature. Testing at temperatures below standard laboratory temperature is not specified. The methods have been used for low-temperature testing, but their reliability under these conditions is not proven.