We have studied the strain relaxation in GaN/Al0.1Ga0.9N superlattices grown by plasma-assisted molecular-beam epitaxy on different buffer layers. A periodic relaxation detected in situ was identified as an elastic phenomenon related to the stress induced by the Ga excess adlayer. Plastic relaxation results in an increase of the density of type a dislocations, which is minimum in the case of growth on GaN. There is no evidence of stacking fault formation or crack propagation in any of the samples. (C) 2010 Elsevier B.V. All rights reserved.