Formaldehyde-urea (F/U) reaction products with molar ratios of 1.8, 2.1, and 2.4 were synthesized at pH 8.3, and the last one also at pH 4.5 using 45%, formaldehyde aqueous solution. For obtaining the resin, the synthesis of F/U 2.1 was continued by acid-catalyzed condensation at pH 4.5 and posttreatment with second part of U (F/U 1.05/1) at 70 degrees C and pH 8.3. The products were analyzed using C-13-NMR spectrometry. Higher excess of F increases the dihydroxymethyl content on account of smaller dimethylene ether content. Certain C-13 chemical shifts in carbonyl and methylene region of spectra were assigned to trishy-droxymethylurea, being the main trisubstituted urea compound in hydroxymethylated product. Acid catalyst promotes the formation of methylene groups by polycondensation of hydroxymethyl groups, against the background of similar content of dimethylene ethers in both catalytic conditions. The ratio of linear/branched chains is emphasized in characterizing the resin structure. Higher hydroxymethyl content in acid-catalyzed polycondensation is an advantage of three-step synthesis technology. The amount of binding methylene and dimethylene ether groups linked only to secondary amino groups can be increased by transhy-droxymethylation with subsequent polycondensation in posttreatment with U in suitable reaction conditions. (c) 2006 Wiley Periodicals, Inc.