Poly(ethylene-co-vinyl acetate) (EVA) is the primary polymer used for encapsulation of photovoltaic (PV) modules. Its degree of cross-linking (gel content) is taken as a major quality reference. The EVA gel content is normally measured by Soxhlet extraction and more recently also by Differential Scanning Calorimetry (DSC). The DSC method is proven here to be fast and effective but is, as the Soxhlet extraction method, destructive to the PV module. With the aim of developing a fast and non-destructive method to determine the gel content, a number of analytical techniques are presented. The most promising method is ultraviolet/visible/near-infrared (UV/Vis/NIR) optical transmission. The measured diffuse transmission reflects the EVA crystallite size, which is related to the EVA gel content. This opens the possibility to apply an in-line analysis of every PV module immediately after the lamination step and could significantly contribute to the process quality control that is needed in future high-throughput production lines of PV modules. Copyright © 2011 John Wiley & Sons, Ltd.