The miscibility of the carbosilane analogue of poly(a-methylstyrene), poly(methylphenyl-silylenemethylene), named poly(sila-alpha-methylstyrene) (PSi alpha MS) with an M(w) value of 188 000 and polystyrene (PS) having different molecular weights is investigated using DSC and cloud point measurements. The observed miscibility behavior is described in terms of the Patterson equation-of-state theory using also PVT data of the respective polymers. Blends with polystyrene of an M, value of 3000 are miscible over the whole composition and temperature range. A further increase of the molecular weights leads to immiscibility for blends with weight fractions of polystyrene more than 0.2 and less than 0.9 for M(w) greater than or equal to 5600 and less than 0.9 for M(w) greater than or equal to 17 500, respectively. The behavior is similar to that of the system poly(a-methylstyrene) (P alpha MS)/PS. However, the system PSi alpha MS/PS is miscible for lower molecular weights only. Obviously, the replacement of the quaternary carbon atom by a silicon atom leads to stronger repulsive interactions. The known miscibility behavior of P alpha MS/PS is described by the Patterson theory as well, in order to compare the interaction parameters of the two systems. Moreover, the temperature dependencies of the chi-parameters of the two systems are compared using different expressions for chi from three equation-of-state theories and based on the Gibbs free energy and the chemical potential. The Flory-Orwoll-Vrij theory, the Patterson theory, and the modified cell model of Dee and Walsh are employed. The interaction of P alpha MS/PS is more favorable to mixing than that of PSi alpha MS/PS. Finally, the experimentally observed immiscibility of PSi alpha MS with poly(2,6-dimethyl-1,4-phenylene oxide) and poly(vinyl methyl ether), respectively is explained in terms of the Patterson theory.