Composting is an aerobic process which allows the rapid proliferation of a variety of microbial groups such as aerobic actinomycetes, bacilli and fungi. Waste mushroom compost is readily available as a byproduct, with 400,000-500,000 tons produced per year in the UK, and is capable of acting as a carrier for the xenobiotic-degrading microorganisms and as a nutrient source in contaminated soils. Bacteria, particularly actinomycetes, were targeted as potential PCP degraders because of their ubiquitous distribution in composts, their ability to colonize composts by mycelial growth and to degrade a wide range of aromatic compounds using either endoor extracellular enzymes (McCarthy and Williams, 1992). Semple and Fermor (1995) showed that actinomycetes, isolated from PCP-contaminated mushroom compost, were capable of attacking this highly chlorinated phenol. However, the individual putative PCP-degrading isolates tested did not achieve appreciable mineralization of UL-14C-PCP. It may be that a diverse population working in concert is required to biodegrade the chlorophenol to CO2 and H2O. However, biotransformation of PCP is a possibility given that the Saccharomonaspora viridis strains, Streptomyces spp., and Thermomonospora spp. appear to remove the PCP from the growth media (Semple and Fermor, 1995). Mineralization of PCP by composts was found to be more impressive compared to that of the actinomycete isolates. This suggests that the physical relationship between compost solid substrate and the populating microflora play an important role in the degradation of PCP. A key phenomenon to be considered when applying a bioremediative technology is chemical ageing and its relationship to bioavailability of the contaminant to the soil microflora (Hatzinger and Alexander, 1995). The longer an organic compound is in a given soil environment, the more likely it is to be affected by chemical and physical processes, such as diffusion into soil micropores, partitioning into soil organic matter, strong surface adsorption and covalent binding to soil constituents. These processes result in a decrease in the bioavailability of the compound to the degradative microorganisms, thus reducing the effectiveness of the treatment. However, it has been suggested that desorption of compounds (previously unavailable), leading to an increase in bioavailability, may occur at elevated temperatures typically generated under composting conditions (Pignatello and Xing, 1996). This will require further investigation to understand the processes involved and to characterize its effects on the degradation of PCP by mushroom composts.