Abstract Heat sealing is widely used in form-fill-seal packaging of liquid food products. One of the problems often encountered by food companies is the contamination of seal area by the liquid product during heat sealing. In this study, heat transfer models were developed to elucidate the heat transfer during heat sealing of two layers of linear low density polyethylene (LLDPE) films, when a liquid contaminant layer is absent or present at the film–film interface. Thermal contact resistance was incorporated in the models to account for the thermal resistance due to the micro-gaps existed on film surfaces and the heat sink effect of the contaminant liquid. The heat transfer models were validated by using experimental film–film interface temperature data when upper jaw temperatures of 155 and 165°C were applied. The models predicted the film–film interface temperatures well with root mean square errors (RMSEs) ranging from 1.6 to 2.1°C for clean seals, and from 1.6 to 2.5°C for water-contaminated seals. Validation results showed that the heat sink effect of water was counterbalanced by the enhanced heat transfer at the film–film interface, probably due to displacement of air in the micro-gaps by the contaminant liquid.