A proton magnetic nuclear resonance study was performed on co-condensation reactions of resorcinol, 5-methylresorcinol and 2,5-dimethylresorcinol with methylol compounds, including ortho- and para-methylolphenol, N-methylol-caprolactam, methylol-N,N-diethylurea, methylolurea and N,N’-dimethylolurea. Spectral assignments, reaction kinetics and composition of products are discussed. The reaction in melt (120 degrees C) with methylolphenols occurs as co-condensation in the presence of all catalysts studied. In resorcinols C4C6 substitution is favored. The rate constants of methylol disappearance clearly indicate the preferable Influence of acid and alkaline catalyst (not zinc acetate) on para-methylol. The reaction with N-containing methylol compound does not give any co-condensate in the presence of alkaline catalyst. The optimum conditions for co-condensation mainly depend on reactivity of co-reagents with formaldehyde and stability of methylol compound. The quantitative amount of co-condensate with methylolcaprolactam can be obtained in melt-condensation (70 degrees C) in the presence of acid catalyst. Because of the higher reactivity of urea, the reaction in melt (100 degrees C) in the presence of acid catalyst does not lead to quantitative co-condensation. The condensation of methylol compound or resorcinols with formaldehyde can be avoided substantially by performing the reaction in aqueous solution at lower temperature.