Solid-phase micro-extraction (SPME) was used for monitoring degradation of hydrocarbons in diesel-fuel-contaminated (1% v/v) water and soil. Natural soil bacteria with and without external addition of inoculum were used. Directly after a 10-s exposure of the sample, the polydimethylsiloxane fibre was injected into the GC-MS. This method strongly reduced the time of analysis compared to a conventional liquid/liquid extraction. A comparison of SPME and pentane extraction of diesel oil was made and found to be consistent. The degradation of diesel fuel in water was monitored for 10 weeks using SPME. After 5 weeks all hydrocarbons were degraded except for the decahydronaphthalenes. These compounds were approximately 3% of the total hydrocarbons in the diesel oil used and remained undegraded throughout the study although none of the chemical or physical parameters was limiting. In the soil study the degradation of diesel fuel in normal soil was completed after 3 weeks, when the only remaining substances were decahydronaphthalenes. All samples were compared to sterile references to make up for evaporation losses. SPME proved to be a fast and reliable method to monitor changes in concentrations of semivolatile organic compounds.