The three-liquid-phase equilibria data of the quaternary system water (1) + tert-butanol (2) + n-decane (3) + n-undecane (4) in the tricritical region are reported. It has been found that the mass fraction of n-decane in (n-decane + n-undecane) 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.