When transporting CO2 for sequestration, it is important to know the water dew point in order to avoid condensation that can lead to corrosion. A flow apparatus to measure the water content at saturation in a compressed gas has been constructed. A saturator humidifies the flowing gas by equilibrating it with liquid water. Then, a gravimetric hygrometer measures the water mole fraction of the humid gas. Dew-point data for H2O in CO2 on six isotherms between 10 and 80 degrees C at pressures from 0.5 to 5 MPa are reported. The uncertainties in water content at the dew point (expanded uncertainty with coverage factor k=2) are on average 0.3%, significantly smaller than in any previous work. The data have been analyzed to extract the interaction second virial coefficient; the values are consistent with the theoretical estimates of Wheatley and Harvey but have a much smaller uncertainty. Published 2015 American Institute of Chemical Engineers AIChE J, 2015 (c) 2015 American Institute of Chemical Engineers AIChE J, 61: 2913-2925, 2015