This paper contains the first results dealing with creosote oils and oilborne preservatives from this in-ground field trial in Australia. The substrates impregnated with preservative were Pinus radiata D. Don sapwood and Eucalyptus regnans F. Muell. heartwood and sapwood. Data are reported from stakes exposed for 35 years at three Australian sites (Innisfail, Sydney, Walpeup). Comparisons were made between preservatives impregnated into P. radiata at 128 kg/m³ and exposed at Sydney. After 35 years, the mean condition of stakes treated with British standard, Australian K.55 (blend) and brown coal tar (high residue) creosote oils were serviceable. The mean condition of stakes treated with USA standard (AWPA P.1), Australian K.55 (ii, old Timbrol) and brown coal tar (distillate) creosote oils was unserviceable (ie. rated 3 or less out of 8). When a proportion of Australian K.55 (blend) creosote oil was replaced by furnace oil, vertical retort tar or 2.5% pentachlorophenol (PCP) in furnace oil, the new combinations did not, on average, rate as highly as the Australian K.55 (blend) creosote oil by itself. PCP was compared at 6.4 kg/m³ in P. radiata at Sydney. 5% PCP in furnace oil (128 kg/m³) performed as well as Australian K.55 (blend) creosote oil and much better than 5% PCP in diesel fuel oil (128 kg/m³). 2.5% PCP in furnace oil (256 kg/m³) rated the highest of any treatment containing 6.4 kg/m³ of PCP. The addition of dieldrin or chlordane improved the efficacy of 2.5% PCP in furnace oil (128 kg/m³) at Sydney, but not at Innisfail. The addition of benzene hexachloride showed greater protection than dieldrin or chlordane.

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