Excess molar enthalpies, excess isobaric molar heat capacities, densities and speeds of sound of the mixtures x(H2O or D2O) + (1-x)CH3CN were measured at 298.15 K. Considerable isotope effects were observed in H-m(E), V-m(E), C-p,m(E), C-V,m(E), K-S(E) and K-T(E). D2O + CH3CN mixture was more positive for the positive excess properties and more negative for the negative ones than those for the H2O + CH3CN mixture. This can be ascribed to the stronger hydrogen bonds of D2O water than H2O water. Excess properties of both mixtures are very similar to each other. H-m(E), values are positive except for the region x > 0.97, where they show a small negative value. C-p,m(E), values are very large and positive, and the trend to more positive values makes an anomalous hump in the region x > 0.8, which corresponds to the negative part of H-m(E). C-V,m(E) values are smaller than those of C-p,m(E) and resemble them. V-m(E), k(S)(E) and K-T(E) values are negative for the whole concentration range.