A membrane osmometer designed for use at pressures greater than 0.1 MPa and less than 6 MPa was employed to determine the pressure coefficient of the equilibrium osmotic pressure (partial derivativepi/partial derivativeP) of a dilute polystyrene/toluene solution. The pressure coefficient of the second virial coefficient (partial derivativeA(2)/partial derivativeP), calculated from partial derivativepi/partial derivativeP, was 6 (+/-4) X 10(-5) cm(3) mol g(-2) MPa-1, which was in reasonable agreement with the value obtained from pressure-dependent light scattering. (C) 2003 Wiley Periodicals, Inc.