The variation of interfacial tension (IFT) with temperature and pressure strongly influences the transport of the fluid in a reservoir. This makes the IFT probably the most important of the factors that make one-third of the total oil in place (in reservoir rock) un-recoverable by gas drive or water flooding [H.Y. Jennings Jr., G.H. Newman, The effect of temperature and pressure on the interfacial tension of water against methane-normal decane mixtures. SPE 1971, pp. 171-175]. Several models and correlations were used to calculate the measured data presented here. We have studied the applicability of four most commonly used IFT-correlations, those of Katz et al. [D.L. Katz, R.R. Monroe, R.P. Trainer, Surface tension of crude oils containing dissolved gases. AIME. Technical publications No. 1624, pp. 285-294.], Hough and Stegemeier [E.W. Hough, G.L Stegemeier, Correlation of surface and interfacial tension of light hydrocarbons in the critical region. SPEJ, December 1961, pp. 259-263.], Lee and Chien [S.T. Lee, M.C.H. Chien, A new Multicomponent surface Tension correlation based on scaling theory. SPE/DOE 12643 fourth symposium on EOR/Tulsa, OK, April 15-18.], and Pedersen et al. [K.S. Pedersen, F. Aage, P. Tomassen, properties of oils and natural gases. Gulf publishing, 1989, pp. 196-207]. The experimental results from the three binary systems as well as the recombined crude oil system have shown that the IFT in the high pressure region (below saturation pressure) remains quite low despite large pressure decrements. This study was then extended to include measurements of the equilibrium interfacial for oil-brine, oil-gas and gas-brine at reservoir temperature and pressure over the range 250 to 3728 psia. The IFT were used to evaluate the spreading coefficient as a function of pressure at reservoir temperature. The viscosity and density of the oil and gas and brine phases were also measured over the same range of reservoir conditions.