A separation into aromatic and nonaromatic molecules was performed on an Indonesian oil mixture with each cut from C-6 to C-45 Each fraction was carefully analyzed using the NMR technique. NMR is not precise enough, however, to exactly determine the group composition of each fraction. Structural hypotheses were therefore developed to improve the efficiency of NMR as an analytical method for use with petroleum fluids. Thanks to group contribution methods developed for this purpose, it has become possible to solve an equation of state and to predict the changes in the relative volume vs the pressure. These results are compared with those obtained using the predictive method recently developed by Neau, Jaubert, and Rogalski. There appears to be little or no advantage to be gained from performing the lengthy, difficult, and expensive NMR analysis.