The near-critical phase behavior of five synthetic reservoir fluid mixtures has been systematically studied using a ROP (France) high-pressure PVT system. The C-7(+)-content in the prepared samples ranges from 6.67 to 11.15 mol%. Through a series of constant composition expansion experiments performed in the temperature range of 303-433 K, the compressibility factor of compressed fluid (in the single-phase region), partial pressure-temperature (P-T) phase envelope, total molar volume and liquid volume fraction in the two-phase region have been measured for each sample. In addition, the critical point data of three samples were determined.