An interferometric method has been used to measure the piezoelectric coefficient d(33) in thin, InN layers grown by remote plasma-enhanced chemical vapour deposition. The measured value of the coefficient varies from around 3.1 to 4.7 pm V-1. Theoretical predictions for these coefficients are scarce, but these values are significantly lower than those which are available. The discrepancy arises largely from the clamping of the film by the substrate, but may also be due to defects within the material. (C) 2004 Elsevier B.V. All rights reserved.