The aim of this study was to verify the hypothesis that a hydrocarbon degrading community isolated from a site heavily polluted with polycyclic aromatic hydrocarbons (PAHs) and heavy metals should exhibit a high activity and biodegradation efficiency, despite decreased biodiversity resulting from the presence of such contaminants. Microbial community isolated from soil collected at an abandoned creosote railway wood-sleepers impregnation plant using diesel oil was used during the studies. Four parallel systems spiked with diesel oil, diesel oil + PAHs, diesel oil + heavy metals and diesel oil + PAHs + heavy metals were analysed in terms of relative abundance and biodiversity of the microbial community (Illumina), biodegradation efficiency (GCMS) and cellular metabolic activity (flow cytometry). Principal Component Analysis and biodiversity parameters indicated that the mixture of PAHs and heavy metals was the dominant factor which resulted in the enrichment of the Gammaproteobacteria class. This was associated with higher degradation of additional PAHs in the presence of heavy metals and an increase of metabolically active sub-populations during flow cytometry analysis. The increased abundance of the Acinetobacter genus in systems with both PAHs and heavy metals implies that it may play a crucial role in soil populations exposed to mixed contaminations.