Forest fires are believed to produce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs); however, there is no firm evidence supporting this conjecture. To address this issue, we investigated whether PCDD/Fs and polycyclic aromatic hydrocarbons (PAHs) are formed by forest fires. The present work takes the indirect approach of measuring the levels of PCDD/Fs and PAHs in soil and ash samples 1, 5 and 9 months after forest fires. To determine if PCDD/Fs or PAHs were formed during forest fires, the levels of PCDD/Fs and PAHs measured in the burnt soil samples were compared with those in the corresponding unburnt soil samples. One month after forest fires, the concentrations of PCDD/Fs and PAHs in burnt soils were higher than in the corresponding unburnt soils. In addition, the homologue profiles differed between the burnt and unburnt soils. Five months after forest fires, however, the concentrations of PCDD/Fs and PAHs in the burnt soils were similar to those in unburnt soils. The data presented here strongly suggest that PCDD/Fs and PAHs form during forest fires and are then introduced into the soil. The results further suggest that the ash resulting from the combustion of wood and other organic matter is the main agent influencing the concentration of PCDD/Fs and PAHs in the soil.