The influence of a high quality compost amendment on two soils contaminated with diesel oil and polycyclic aromatic hydrocarbons, PAHs, respectively, was evaluated, with respect to contaminant biodegradation, microbial community composition and soil toxicity and genotoxicity. For each of the two soils, two 20-kg biopiles were set up, one without amendments and one compost-amended. GC/FID analyses revealed that compost was effective in enhancing biodegradation of diesel oil and of four-ring PAHs. It also influenced microbial community composition, as inferred by ARDRA analyses and partial sequencing of 16S rDNA of clones from libraries constructed from each soil sample. Microtox analyses on soil aqueous saline extracts and Solid Phase Tests showed some toxicity reduction due to compost addition, while the Comet assay, performed on coelomocytes of earthworms exposed to contaminated soils, did not show genotoxicity reduction. In general, the use of compost amendment to hydrocarbon-contaminated soils in a bioremediation process proved to be effective for depletion of contaminants and reduction of toxicity.