AbstractUrea, thiourea and someN′‐alkyl(di)substituted ureas and thioureas were found to react as the amine component in Mannich‐type condensation reactions withp‐toluene‐ or phenylmethane‐sulfinic acid and aldehydes. Mono‐ or di‐alkylation products are formed in good yields depending on theN′‐ andN′‐substituents in the substrate, whileN,N,N′‐trimethylurea or ‐thiourea failed to react.N′‐Hydroxy‐N′‐phenylthiourea is alkylated on theN′‐atom bearing the OH‐substituent as demonstrated by the ESR spectrum of the thioacyl nitroxide derived from the condensation product. Thiobenzamide, in contrast with thioacetamide, also condensed withp‐toluenesulfinic acid and formaldehyde orN′‐butyraldehyde. With benzaldehyde, however, Mannich‐type condensation did not occur and 3,5‐diphenyl‐1,2,4‐thiadiazole was isolated instead.The NMR spectra of several urea and thiourea condensation products provided evidence for hindered internal rotation in the urea and thiourea function and some ΔG≠values for the exchange process were estimated. Intramolecular shielding effects were observed in the NMR spectrum