Oils from Guafita field (Barinas basin) were studied using two common spectroscopic techniques for liquid or viscous samples: transmission and attenuated total reflection (ATR). The obtained spectra were similar and there was an excellent correlation between compositional indexes calculated for oils analyzed by both techniques. When areas are integrated a large variability appears, related to band intensity and overlap. Oils from Guafita field show similar aromaticities, but present differences in the aliphaticity and branching, which could reflect secondary processes that took place in the reservoir.Projects CDCH-UCV, Venezuela (PG-03-00-6518/2006) and LOCTI (TOTAL and REPSOL).