Copper-ethanolamine based wood preservatives are replacing classical copper-chromium based solutions. Fixation of copper-ethanolamine based wood preservatives is not as good as fixation of copper-chromium ones. We presumed ethanolamine to cause depolymerization of lignin, what consequences in increased leaching. To find the reason for insufficient fixation of copper-ethanolamine formulations, Norway spruce (Picea abies) and beech (Fagus sylvatica) wood specimens were impregnated with 3 different aqueous solutions consisting of copper-ethanolamine and additives/co-biocides of 3 different concentrations. After fixation, those specimens were leached; afterwards the mass change was gravimetrically determined. Modulus of elasticity (MoE) was determined using non-destructive technique; copper, nitrogen and carbon contents in leachates were analysed. Carbon and nitrogen contents in leached and un-leached wood blocks were measured as well. The results show that the presence of ethanolamine in preservative solutions causes MoE change. In leachates, significantly increased carbon as well as copper contents was determined. There was good correlation between organic carbon in leachates and copper leaching. The main source of organic carbon was wood components. Therefore, we believe that our hypothesis of ethanolamine induced lignin depolymerization is confirmed.