A hydrocarbon mixture containing p-xylene, naphthalene, Br-naphthalene and straight aliphatic hydrocarbons (C-14 to C-17) was aerobically degraded without lag phase by a natural uncontaminated potting soil at 20 degrees C and 6 degrees C. Starting concentrations were approximately 46 ppm for the aromatic and 13 ppm for the aliphatic compounds. All aliphatic hydrocarbons were degraded within 5 days at 20 degrees C, to levels below detection (ppb levels) but only down to 10% of initial concentration at 6 degrees C. Naphthalene was degraded within 12 days at 20 degrees C and unaffected at 6 degrees C. At 20 degrees C p-xylene was degraded within 20 days, but no degradation occurred at 6 degrees C. Br-naphthalene was only removed down to 30% of initial concentration at 20 degrees C, with no significant effect at 6 degrees C. The biodegradation was monitored with head space solid-phase microextraction and chromatography-mass spectrometry.