This paper reports on a unique crystallization phenomenon of amorphous plasma-enhanced chemical vapour deposited SiNx thin films induced by heating to temperatures >= 1148 K in air. The crystallization occurs as randomly scattered pits, within which clusters of smaller crystals form. The crystals formed are free of oxygen and are identified to be alpha-Si3N4. The crystallization is associated with a large volume contraction, which results in internal cracking. Crown Copyright (c) 2007 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. All rights reserved.