Phase-transition extraction (PTE) is a recently developed separation technique using partially miscible solvent systems that demonstrate temperature-dependent phase splitting behavior. This separation technique exploits the ability to shift solvent systems across their liquid-liquid coexistence curves, alternating between homogeneous and two-phase systems to achieve the desired separation. This work examines the phase behavior of fluorinated-hydrocarbon solvent systems and the ability to manipulate their critical solution temperatures (CSTs) with the addition of a third component. In addition to PTE, knowledge of the liquid-liquid equilibria (LLE) of these systems has applications to biphasic and homogeneous fluorous phase chemistry and the improved recycling and recovery of fluorous solvents. The LLE of ternary systems containing a hydrofluorocarbon, Vertrel-XF, n-decane, and either n-hexane or 1-octene were determined in the temperature range of 273-323 K. The ability to manipulate the CST of Vertrel-XF + n-hexane and Vertrel-YF + 1-octene with the addition of n-decane was examined. Experimental data were correlated using the UNIQUAC and NRTL activity coefficient models. These models were capable of representing the temperature-dependent types I and II phase behavior observed for these systems.