The densities of an artificial estuary formed by mixing artificial average river water with standard seawater have been measured with a vibrating densimeter at 25¿C. The densimeter was calibrated by measuring the densities of weight evaporated and diluted (with pure water) standard seawater solutions. The measured seawater densities agree with those determined from the equation of state of Millero et al. (1975a) to ¿2 ppm. The densities d of the artificial river waters were found to be in good agreement (4.2 ppm) with those determined by using apparent equivalent volume data for the major river components: d=d0+10-3(M-d0&PHgr;v) N, where d0 is the density of pure water, N=&Sgr;ee is the normality (eq 1-1), M=&Sgr;EtMt is the mean equivalent weight, and &PHgr;EtMt is the mean equivalent weight, and &PHgr;V=&Sgr;EtϕV(i) is the mean apparent equivalent volume of river salts (Et=et/N is the equivalent fraction, Mt is the equivalent weight, and ϕv(i) is the apparent equivalent volume of ionic component i). The densities of the artificial river and the estuarine waters were found to be in excellent agreement (¿1.6 ppm) with the densities of seawater solutions (diluted with pure water) at the same total solid concentration or salinity. These results confirm the theoretical calculations of Millero (1975a, b, c) and indicate that the densities of rivers and estuaries can be determined from the equation of state of seawater diluted with pure water (Millero et al., 1975a).