Biocide-containing nanoparticles were suspended in water to prepare wood treating suspensions able to deliver up to 0.8 kg biocide/m(3) of wood. The nanoparticle preparation method was versatile, and three fungicides (tebuconazole, chlorothalonil, and KATHON 930) and one insecticide (chlorpyrifos) were incorporated into the nanoparticles with little customization of the preparative method. Greater customization was required when the polymer matrix was changed, but the method was generally robust; nanoparticles could be prepared from several different polymers, copolymers, and polymer blends. The median nanoparticle size increased as the matrix hydrophobicity increased. Nanoparticles were quantitatively delivered into birch and southern yellow pine (SYP) at low suspension loadings, but the delivery efficiency decreased with increased suspension loading and with increased matrix hydrophobicity. The delivery efficiency was also less for birch than for SYP. Undelivered nanoparticles were found to have undergone aggregation. Greater aggregation occurred in the more hydrophobic formulations than in the hydrophilic formulations. High biological efficacy was found for all the biocides tested. Nanoparticle-treated birch was exposed to Trametes versicolor for 55 days and some protection was afforded, even at biocide loading levels of only 0.1 kg/m(3). At the highest loadings (similar to0.6 kg/m(3)) the weight loss after exposure to T. versicolor was generally similar to10% for most formulations. The SYP was treated with KATHON 930 in polyvinylpyridine. At levels of 0.1 kg of biocide/m(3) of wood less than 5% of the SYP mass was lost after 50 days of exposure to Gloeophyllum trabeum.