The miscibility of four binary liquid mixtures containing acetonitrile (ACN, ACN-d(3)) and water (H2O, D2O) has been studied. All the measured systems present solubility curves characterized by slight asymmetry and the presence of an upper critical solution temperature (UCST). It is found that all systems have approximately the same critical composition and approximately the same shape of the phase diagrams. There is also no influence on the critical exponent. However, it has been found that isotope substitution diminishes solubility in each case independently of the site of deuterium substitution. This observation is accounted for in terms of the specific interactions of acetonitrile and water.