Cross-sectional TEM and SEM have been used to study the degradation of organic solar cells when exposed to a high humidity environment. Two obvious effects were caused by this exposure: voids at the aluminium/calcium interface and large bubble like protrusions on the top surface. Water ingress was found to occur mainly from the edge rather than through pinholes or defects in the aluminium film. The grain size of the aluminium used in the cathode was varied and small grained devices were found to have degraded faster. The importance of minimising water ingress into devices through cathode design and choice of materials is highlighted. (C) 2015 The Authors. Published by Elsevier B.V.