The occurrence and distribution of aromatic hydrocarbons in oil samples from an oilfield in the NW Niger Delta was investigated, focussing on naphthalenes, phenanthrenes and mono- and triaromatic steroids. Tetramethylnaphthalene is the most abundant of the naphthalene homologues, while dimethylphenanthrene is the most abundant of the phenanthrene homologues. The relative concentration of the sum total of the phenanthrenes and their isomers was greater than that of the naphthalenes. The distribution of naphthalenes and phenanthrenes was strongly controlled by thermal maturation and biodegradation of the oils. Various aromatic hydrocarbon- dependent geochemical parameters indicate that the oils are mature and were generated at similar levels of thermal maturation. Plots of these parameters showed that the distribution of homologues of both naphthalene and phenanthrene is sensitive to the biodegradation of the oils. Infrared spectroscopy (IR) was used for geochemical characterisation of the NW Niger Delta oil samples in terms of both source type and thermal maturity. The characterisation was based on the peak intensities of the aliphatic and the carbonyl groups relative to the aromatics. The study showed that IR may be useful for the determination of thermal maturity, but is not a good tool for source characterisation.