Abstract Eleven new ureas of the type, 1,1-diethyl-3-arylureas and 1-pentyl-3-arylureas were synthesized. The rate constants and ρ values of thermal dissociation of these ureas in fatty acids were determined. In 1,1-diethyl-3-arylureas, the ρ value of the dissociation reaction at 95°C. in chloro-acetic acid was −0.196, and it may be concluded that the protonation process seems to be the rate-determining step in the dissociation reaction of ureas in fatty acid. The activation energy was compared with those of other ureas.