The PVTx properties of H(2)O + n-C(5)H(12) mixtures have been measured in the near- and supercritical regions. Measurements were made along 66 liquid and vapor isochores in the density range from (63 to 713) kg center dot m(-3), at temperatures from (303 to 684) K, and at pressures up to 63 MPa. Measurements were made for eight concentrations between 0.0270 and 0.8898 mole fractions of n-C(5)H(12). The temperatures and pressures at the three-phase (liquid liquid gas) and two-phase (liquid gas) boundary curves for the H(2)O n-C(5)H(12) mixtures were obtained using the isochoric (P T) break point technique. The expanded uncertainty of the density, pressure, and temperature measurements at the 95 % confidence level with a coverage factor of k = 2 is estimated to be 0.12 % (at high densities) to 0.15 % (at low densities), (0.0005 to 0.03) MPa, and 15 mK, respectively. The critical property (P(C,)T(C)) data of the upper and lower branches of the critical curves were extracted from the derived phase-boundary data. The measured three-phase data were used to estimate the value of the upper critical end point. The value of the Krichevskii parameter was calculated from the direct measured PVTx and the derived critical property data.