A new method is introduced to determine the absolute value of the boundary excess free volume. Along with the boundary energy the excess free volume belongs to the major thermodynamic properties of grain boundaries. The method utilizes the dependence of the contact angle at triple junctions of grain boundaries in Al-tricrystals on hydrostatic pressure. We investigated < 111 > tilt boundaries in the pressure range up to 14 kbar. In particular, for a 400 < 111 > tilt boundary with 2 degrees twist component the boundary free volume was found to be equal to 5.03x10(-11) m(3)/m(2).