The electrical conductance of aqueous solutions of HCl has been measured at 298.15 K as a function of (18)O-enrichment of water and can be linearly extrapolated to isotopically pure (1)H(2)(18)O. The high proton mobility in normal water ((1)H(2)(16)O) is retained in (1)H(2)(18)O. The conductance reduction by complete (18)O/(16)O substitution by a factor of 1.050 approximately corresponds to the increase in solvent viscosity and is substantially lower than the reduction in the water + deuterium oxide ((2)H(2)(16)O+(1)H(2)(16)O) system. The nonlinear dependence of the conductance of (2)HCl/(1)HCl upon the degree of deuteration in mixtures of normal water and deuterium oxide has no analogue in mixtures with (1)H(2)(18)O.