The co-condensation between resorcinols, including the most typical components of oil shale alkylresorcinols (5-methylresorcinol, 2,5-dimethylresorcinol), and methylol-N,N-diethylurea or monomethylolurea is the favoured reaction in the presence of acid catalyst. Methylene H-1 chemical shifts ape assigned and the integrals of signals used for determination of co-condensate methylene molar concentration. C4 monosubstituted and C4C6 disubstituted resorcinols are the predominant structural elements of co-condensates.