The biotreatability of a xenobiotic contaminated soil is frequently determined through a bioslurry treatment usually performed in lab-scale shaken baffled flasks. In this study, a 3-1 unconventional stirred tank reactor was developed and tested in the slurry-phase treatment of a soil heavily contaminated by polychlorobiphenyls (PCBs) derived from an Italian dump site, in the absence and in the presence of biphenyl and of the exogenous PCB aerobically dechlorinating co-culture ECO3. The data obtained were compared with those obtained on the same soil in experiments performed in parallel in 3-1 baffled shaken flask reactors. Considerably higher PCB removal and soil detoxification yields (determined through the Lepidium sativum germination test and the Collembola mortality test) were attained in the stirred tank reactors? which generally displayed a higher slurry-phase homogeneity and a higher availability of biphenyl- and chlorobenzoic acid-degrading bacteria compared to the corresponding shaken flask reactors. Moreover, enhanced soil PCB biodegradation and detoxification yields were observed when the developed reactor was supplemented with biphenyl and the exogenous ECO3 bacteria. In conclusion, the results of the soil biotreatability experiments commonly performed in bioslurry lab-scale reactors are significantly influenced by the reactor configuration; the use of the unconventional stirred tank reactor system developed in this work is recommended.