Abstract The purpose of this study was to compare the reaction chemistry of micronized copper and alkaline copper treatments with wood and to determine how fast copper is solubilized during the reaction between the acidic functionality in wood and the basic copper carbonate (CuCO3). Copper species produced in wood by various treatment methods were analyzed by electron paramagnetic resonance (EPR) spectroscopy. The effects of time and solution concentration on the spectral parameters of copper complexes in sawdust treated with copper sulfate solutions were examined, followed by study on the structure and the rate of formation of the copper complexes in sawdust treated with basic CuCO3 suspension. The study further attempted to identify the soluble copper species formed in wood during treatment with micronized copper preservatives. Comparisons were made among the fixed copper complexes in wood treated with micronized copper preservatives, copper sulfate solutions, basic CuCO3 suspensions and alkaline copper solution. The results showed rapid formation of fixed copper complexes in wood treated with aqueous suspensions of basic CuCO3. These complexes can resist leaching, and they have similar stereochemistry to those formed between wood and copper sulfate. This finding supports the premises that soluble copper is generated during the treatment of sawdust with basic CuCO3, and it can bond to wood cell components by migrating into the cell wall in a manner similar to other soluble copper species. Such copper complexes formed are different from those of alkaline copper treated wood, which can be easily distinguished by EPR.