Post-compression CO2 delivered to sub-surface reservoirs often contains various impurities including sub-saturated water, where understanding phase behavior or chemical reactions involving water requires volumetric information. A new optically coupled vibrating tube densimeter has been developed for volumetric measurements of CO2 injectate fluids for compression conditions and injectate transportation (temperatures less than 423 K and pressures less than 35 MPa). Details regarding the design, operation and calibration of the instrument are reported. Density differences for sub-saturated H2O in CO2 from T= (318 to 369) K and up to p = 35 MPa (m = (0.053 to 0.065) mol . kg(-1)) are reported and used to calculate molar volumes. To our knowledge, these are the first sub-saturated measurements to be reported at the conditions typical for CO2 compression and transportation. As expected, the apparent molar volumes become significantly negative near the CO2 supercritical conditions. Using these molar volumes, we have (i) provided a fitted equation based on Fluctuation Solution Theory which can be used to calculate H2O fugacity coefficients at infinite dilution, (ii) found the Krichevskii parameter to be A(kr) = (17.7 +/- 3.0) MPa and (iii) used the volumetric data to optimize reduced Helmholtz energy mixing parameters which can be used with high-accuracy equations-of-state (gamma(upsilon,12) =1.03070 and gamma(T,12) = 0.81776). Comparisons to previously reported data for the H2O-rich phase and the recent mixing model of Gernert and Span show the resulting corresponding state mixing parameters can be utilized for a much larger range of H2O concentrations, temperatures up to T= 673 K and pressures up top = 100 MPa. (C) 2015 Elsevier Ltd. All rights reserved.