High protein dairy beverages are considered to be mouth drying. The drying sensation may be due to the product protein content; however the mechanism of this mouth drying is uncertain. This study investigated the potential adhesion of milk proteins to porcine oral mucosa in vitro. Purified casein and β-lactoglobulin were fluorescently labelled, placed on porcine oral mucosal tissues and their resistance to wash out with simulated saliva was monitored using fluorescence microscopy. Casein was found to be more adhesive to porcine mucosa than β-lactoglobulin. Some investigation into the reason for this difference in mucoadhesion was conducted by thiol-content analysis, rheology and zeta-potential measurements. The higher viscosity of casein solution and smaller zeta-potential is believed to be responsible for its better retention on mucosal surfaces. These findings suggest that casein and whey protein are both capable of binding and eliciting mouth drying in high protein dairy beverages.