Muon spin relaxation has been used to study the dynamics of hard domains in three polyurethane elastomers based on methylenediphenyleneisocyanate. Repolarisation curves of the muonium spin show the muon implants to the aromatic ring of the hard segments. Qualitative changes in the muon spin relaxation rate indicative of phenyl ring dynamics can be seen as the polyurethane is heated from 100 to 470 K. Interpretation of the changes seen in the muon spin relaxation rate are hampered by the absence of a detailed theoretical understanding. Further, poor muon counting statistics of this initial experiment make an accurate analysis impossible. (C) 2000 Elsevier Science Ltd. All rights reserved.