The effect of branching on miscibility has been studied in model systems of chlorinated branched polyethylenes. The branched polymer molecule was represented as a copolymeric chain composed of end, linear interior, and branch-point segmental units, each of which could be either chlorinated or unchlorinated. An expression was derived for the overall interaction parameter for blends of the branched chlorinated polyethylenes based on counting intra- and interchain pair interactions between segmental units. Theoretically, when branching is unfavorable for mixing, the miscibility regime in a branched polymer/branched polymer blend (as represented in an isothermal copolymer-copolymer composition plot) is smaller than that of the corresponding linear polymer/linear polymer blend. When branched polymers are mixed with linear polymers, the symmetry of the miscibility regime in such a plot found with branched/branched or linear/linear blends disappears. The theoretical development was used in an analysis of the phase behavior of 50/50 wt % blends of amorphous CPE(B)/CPE(B), CPE(L)/CPE(SB), etc., where CPE(L), CPE(SB), and CPE(B) represent chlorinated linear, short-chain branched, and long-chain branched polyethylene, respectively. Miscibility of these blends was studied by using standard differential scanning calorimetry. The results support the theory qualitatively and show that the CPE(L) and CPE(SB) present similar miscibility behaviors because the short-chain branches only slightly disfavor mixing. However, CPE(B) generally has more limited miscibility; the long-chain branches have a rather profound effect on mixing.