Abstract Void formation as a function of resin moisture content was investigated to better understand and control process defects in composite parts made from prepreg. In this study, uncured prepreg was conditioned at 70%, 80% and 90% relative humidity and at 35 °C. Conditioned prepreg was laid up into quasi-isotropic laminates and cured using vacuum bag only (VBO) processing (low-pressure), and autoclave processing. Moisture uptake in the resin was measured using coulometric Fischer titration. Void content was measured by image analysis of polished sections of cured laminates. Void fractions increased substantially with increasing moisture content in VBO processed laminates, while autoclave-processed parts remained void-free. Experimental results were consistent with trends predicted using a diffusion-based analytical model. The findings are discussed in the context of causes of voids in prepreg composites.