The isochoric PVT behavior of two binary fluid mixtures has been studied over extensive ranges of temperature and pressure. Emphasis in both experimental investigations was on definition of single-phase compressed liquid behavior, as indicated by isochoric curvatures and isochoric inflection loci, although PVT behavior in the virial region was also determined with comparable accuracy. In the first study, a mixture of 4.65 mole % hydrogen and 95.35 mole % methane was studied at temperatures from 140 to 273.15 K, densities from 2.2 to 23.6 moles/liter, and pressures to 700 bar. Densities of the nine isochores were determined by coupling to a reference isotherm at 273.15 K, where isothermal Burnett experiments were conducted to determine the dependence of density on pressure. Results of this hydrogen-methane study showed that the mixture isochoric inflection locus is "open". This is surprising, as it qualitatively resembles the inflection locus of hydrogen more than that of methane, even though hydrogen is the minority constituent of the mixture. This has significant implications for prediction of hydrogen-methane mixture properties by corresponding states and equation of state methods. Conventional mixing rules cannot predict the quantitative and qualitative dominance of hydrogen at such low concentrations. The second isochoric study consisted of measurements on a mixture of 20.19 mole % methane and 79.81 mole % methanol. Thirteen isochores were measured at temperatures from 350 to 550 K, densities from 1.7 to 21.8 moles/liter, and pressures to 700 bar. A reference isotherm at 510 K was used to determine isochore densities. A gravimetric technique was used to characterize this isotherm. Methanol decomposition at the high temperatures of this study was modeled as a pseudohomogeneous zeroth-order reaction. Each isochoric data point was density-corrected using the model parameters. Results of the methane-methanol investigation show the mixture isochoric inflection locus is "open", like pure methanol. The exact shape of the locus could not be determined because points on the locus were outside the operating range of the apparatus and/or methanol's stability region. Strong positive curvature near the bubble point line at high densities indicates the "open" nature of the locus.