The pressure dependence of solution pH for acetate, cacodylate, phosphate, and TRIS buffers in deuterium oxide solutions was measured with the optical pH indicators 2,5-dinitrophenol and p-nitrophenol up to a pressure of 300 MPa. The results give the pressure dependence of the weak acid ionization constant for these buffers in D2O solution. Neuman et al. [J. Phys. Chem. 1972, 77, 2687] measured the pressure dependence of K-a for these buffers in H2O solutions and used the equation RT ln(K-p/K-0) = -Delta V-a degrees P + 0.5 Delta kappa degrees P-2 to describe the observed results where Delta V-a degrees is the limiting volume change on ionization at atmospheric pressure and Delta kappa degrees reflects the pressure dependence of Delta V-a degrees. The present experiment shows that there is no significant deuterium isotope effect on the pressure dependence for these weak acid ionization constants.