Using densitometry of the partial molar volume in dilute aqueous solution at 25 °C, the genetic variant A of bovine β-lactoglobulin was found to be larger than the genetic variant B by 101.0 mL mol−1. Moderate high hydrostratic pressure increased the partial molar volume of the A-variant and B-variant up to 1.5% and 2.0%, respectively, with 30 min holding time. The observed changes in volume were not related to thiol/disulphide exchange and polymerisation reactions, but were a consequence of an increasing water binding at the protein surface in balance with an increasing water binding of the exposed protein interior. The effect was larger for the A-variant in agreement with its lower denaturation temperature and less robustness against enzymatic hydrolysis. Pressure denaturation of the β-lactoglobulin variants occurs, despite pressure-induced protein volume increase, resulting from an increase in water binding with an overall decrease in reaction volume for the denaturation process.