The effects of an organic biocide (CTN) with and without co-added antioxidant (BHT) on microbial communities in SYP were assessed using terminal restriction fragment length polymorphism (T-RFLP) analyses in both field and accelerated decay laboratory studies. Ammoniacal copper quaternary (ACQ-C) was used as a positive control in the field study component, but not in the laboratory test. Field stakes were treated with 0.25 and 0.37% ammoniacal copper quat (ACQ-C), CTN (0.1 and 0.25%), CTN (0.1 and 0.25%) with 2% BHT added, 2% BHT alone, and controls were left untreated. In the field studies, preservative treatment slowed the initial colonization of wood by fungi. Higher species richness and diversity were found in non-biocidal treatments (BHT and untreated controls). Fungal communities in treated wood were different based on their species composition, but eventually became more similar to untreated controls. Preservative treatment increased richness and diversity of basidiomycete fungi, but overall presence of basidiomycetes was low compared to other fungi. Preservatives did not change the species composition of basidiomycetes compared to untreated controls. Preservative treatment initially increased bacterial richness and diversity, but over time these trends diminished to levels consistent with untreated controls. Preservatives changed the species composition of colonizing bacteria so that treated and untreated communities remained different over 15 months of soil exposure. Bacterial diversity was negatively correlated with CTN depletion at the lowest rate. In the accelerated decay laboratory test, the effects of CTN and/or BHT on bacterial, fungal, and basidiomycete communities in composted and uncomposted soil were evaluated over a 12 month period. Composted soil had less fluctuation in changing microbial diversity due to more constant moisture. The consensus of the analyses of the bacterial, fungal, and basidiomycete communities indicate that wood preservatives increased microbial species richness and diversity. Preservative treatment increased species turnover that decreased over time. Eventually, microbial communities approached a stable community structure consistent with untreated controls. Preservatives were completely degraded after 30 days exposure; however, definite changes in bacterial and fungal richness, diversity, and species composition were found. Basidiomycetes again represented the smallest portion of the microbial community involved in the overall decay process.