The impacts of increases in atmospheric CO₂ since the midst of the 18th century on average seawater salinity and acidity are evaluated. Assuming that the rise in the planetary mean surface temperature continues unabated, and that it eventually causes the melting of terrestrial ice and permanent snow, it is calculated that the average seawater salinity would be lowered not more than 0.61‰ from its current 35‰. It is also calculated –using an equilibrium model of aqueous carbonate species in seawater open to the atmosphere- that the increase in atmospheric CO₂ from 280 ppmv (representative of 18th-century conditions) to 380 ppmv (representative of current conditions) raises the average seawater acidity approximately 0.09 pH units across the range of seawater temperature considered (0 to 30°C). A doubling of CO₂ from 380 ppmv to 760 ppmv (the 2 × CO₂ scenario) increases the seawater acidity approximately 0.19 pH units across the same range of seawater temperature. In the latter case, the predicted increase in acidity results in a pH within the water-quality limits for seawater of 6.5 and 8.5 and a change in pH less than 0.20 pH units. This paper's results concerning average seawater salinity and acidity show that, on a global scale and over the time scales considered (hundreds of years), there would not be accentuated changes in either seawater salinity or acidity from the observed or hypothesized rises in atmospheric CO₂ concentrations.