13C solid-state NMR has been used for quantitative determination of the proportion of various functional groups present in uncured urea-formaldehyde (UF) resins. the quantitative interpretation of the structural units present in UF resins synthesized in different pH conditions was made possible by using appropriate rotors and pulse techniques. Resin prepared in acid medium showed more methylene units in the polymer branch and less methylol moieties in the polymer chain compared to resin prepared in alkaline condition. Plausible mechanistic routes are proposed to explain the structural differences due to pH change.