The three-liquid-phases equilibria data of the quaternary system water (1) + formamide (2) + cyclohexane (3) + diethylene glycol monobutyl ether (4) in the tricritical region are reported. It has been found that the mass fraction of formamide in (formamide + water) controls the size of the three-phase region and the approach to the tricritical point. The experimental data have been analyzed to obtain the tricritical temperature and the tricritical composition.