Carpet yarn stability is important with respect to the appearance retention, abrasion resistance and tuft definition of the carpet pile. Various methods of measuring yarn stability have previously been investigated but all have given difficulty when attempting to differentiate between yarns with a high degree of stability i.e. felted yarns. This paper details a new and rapid method of determining the degree of structural stability in set and felted yarns. Operation of the equipment is described, and detailed procedural examples show that the testing apparatus is sufficiently flexible to differentiate between differently felted samples and conventionally set yarns. The effects and relevance of the test variables are described and the use of the IWS Yarn Stability Tester for quality control purposes is discussed.