At present, the position of the authorities in the European Union is not clear on whether to accept natural attenuation (NA) as a strategy to manage contaminated sites. A key issue is that destructive processes, such as biotic and abiotic degradation, must prevail over non-destructive processes (e.g. sorption, diffusion, dispersion) at a potential NA site. This study identifies naturally occurring degradation processes in the subsurface of a former oil refinery (1927–1960), a site that comprises crude oil and its products as major contamination in both soil and groundwater. Attenuation of the contaminant that comprises mainly aliphatic hydrocarbons, is indicated by the depletion of electron acceptors in groundwater. To localize attenuation activities in the plume, geochemical field data are processed using multivariate statistics, and groups of similar degradation characteristics are identified. Results show that considerable degradation of hydrocarbons is proceeding at the site, using oxygen, nitrate, sulphate and ferric iron as electron acceptors. Nitrate and sulphate turned out to be the major electron acceptors for hydrocarbon biodegradation, with an annual degradation of 119 kg n-eicosane. However, no long-term attenuation scenario can be predicted from present data, as anaerobic degradation kinetics for aliphatic hydrocarbon mixtures are not yet fully understood.