The kinetics of the nitrosation of 1,3‐dimethyl (DMU), 1,3‐diethyl (DEU), 1,3‐dipropylurea (DPU), 1,3‐dibuthyl (DBU), and 1,3‐diallylurea (DAU) were studied in a conventional UV/vis spectrophotometer in aqueous‐perchloric acid media. The kinetic study was carried out using the initial rate method. The reaction rate observed was where K_a is the acidity constant of nitrous acid. The diureas exhibited the reactivity order DMU ? DEU > DPU > DAU, which can be interpreted as a function of the steric impediment generated by the R alkyl group in the rate controlling step. A probable relationship between both the chemical reactivity and structure of the nitrosable substrate with the biological activity of the N‐nitroso compounds generated is proposed. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 273–279, 2004