Polymer softening is a major problem in industrial gas phase polymerizations, because it enhances particle agglomeration and reactor wall fouling. It is thus desirable to investigate this phenomenon and identify safe operating conditions. We developed a new experimental method for the characterization of polyolefin softening and proposed a theory qualitatively correlating this phenomenon with the Young modulus. The developed laser dilatometry technique is based on the measurement of polyolefin powder deformations under a constant load at elevated temperatures. Our method enables to measure the softening of polyolefin powders in air and in hydrocarbon gases/vapors, which means that polyolefin softening can be studied at industrially relevant conditions. In the experimental part, we present results obtained for a broad set of polyethylene samples in various atmospheres. The results are in very good agreement with our theoretical expectations and thus verify the proposed effects of polymer properties and penetrant sorption on the softening of polyethylene. (C) 2013 Elsevier B.V. All rights reserved.