Aerobic biodegradation of C5+, a contaminant at ethane pyrolysis plants, consisting mainly of benzene, toluene, xylene, styrene, dicyclopentadiene (DCPD) and naphthalene, removes all components except the recalcitrant DCPD. The number (N) of total culturable heterotrophs in contaminated and uncontaminated soil cultures increased up to three orders of magnitude upon incubation with C5+, or with C5+ components other than DCPD. Exposure of soil cultures to C5+ for 2-4 weeks and to DCPD for a further 43-52 weeks caused N to increase 3, then decrease two orders of magnitude. Early microbial communities (up to 13 weeks) were dominated by Pseudomonas spp. and late communities (40-60 weeks) by Alcaligenes spp., especially in contaminated soil cultures. The hydrocarbon used (C5+ or one of its components) was a less important determinant of community composition than incubation time. Rates of aerobic degradation of benzene, toluene and styrene by the soil cultures were greater for the pure components than for the components in the C5+ mixture. The difference was less pronounced for m-xylene. The exception, naphthalene, was degraded more slowly when in pure form. However, the combined rate of all C5+ components in the mixture was similar to the highest rates observed for pure components. We conclude that soil communities efficiently metabolize a C5+ mixture or its pure components and that the community composition is hardly affected by different hydrocarbon inputs. Consequently, determination of the community composition does not provide a good indicator of the hydrocarbons being metabolized.