Copper remains the primary biocide component used today to protect wood. Increased interest in the use of non-arsenic copper-based wood preservatives has also led to increased studies on copper-tolerant decay fungi. Oxalic acid production by brown-rot fungi is proposed as one mechanism of copper tolerance. This study evaluated oxalic acid production and copper losses from ACQ- and CCA-treated Pinus sylvestris L. and Populus x euramericana 1 214 wood by brown-rot fungi: Postia placenta, Gloeophyllum trabeum, Tyromyces palustris, and 2 strains of Serpula lacrymans. There was no clear linear correlation among mass losses, oxalic acid production, or copper losses in most cases. However, T palustris and one strain of S. lacrymans showed copper tolerance in treated wood. P placenta caused considerable mass losses in ACQ-treated specimens only. G. trabeum produced very low oxalic acid and was inhibited by ACQ and CCA wood preservatives. We concluded that wood species and preservative formulation affected the oxalic acid production, mass losses, and copper tolerance of the tested fungi.