The influence of glutaraldehyde blend on the properties of a commercially available resin in moist conditions was investigated. A UF resin with very low water resistance was blended with glutaraldehyde prior to the thermosetting step and the adhesive performance of the modified adhesive systems was studied. Experiments were performed on birch two-layer co-oriented plywood. It was shown that blending a commercial glue-mix as sold with glutaraldehyde can be an effective way to improve water resistance of the glue without introducing additional -NH2 moiety. It was also found that glutaraldehyde does not deteriorate properties of the glue in the dry state, while glue susceptibility to boiling water is markedly reduced.