A compositionally symmetric random copolymer of styrene and methyl methacrylate (S-r-MMA) was previously found to exhibit preferential miscibility with poly(methyl methacrylate) (PMMA) as compared with polystyrene (PS). This asymmetric miscibility of a random copolymer with its related homopolymers is not predicted by the Copolymer Effect theory, and its underlying causes are unclear in the absence of strong intermolecular interactions. In this paper, correlations were explored between asymmetric miscibility in random copolymer/homopolymer blends and the physical characteristics of the monomeric units. Four copolymer systems were studied, including S-r-MMA, which systematically varied the size and polarity of the monomeric units. A new technique, cross-sectional microscopy of thick bilayers, was developed to determine coexistence compositions in these blends based on linearized mass balances. In addition to S-r-MIMA, a second compositionally symmetric random copolymer, poly(styrene-mn-ethyl methacrylate), was found to exhibit preferential miscibility with poly(ethyl methacrylate) over polystyrene. Two other copolymer systems, poly(styrene-ran-butyl methacrylate) and poly(styrene-ran-4 vinylpyridine), exhibit comparable miscibility with their related homopolymers. Comparing the monomer characteristics to both our miscibility and infrared spectroscopy results suggests that differences in the size and polarity of the monomeric units may induce asymmetric miscibility in random copolymers. Finally? deuteration within the S-r-MMA system was sufficient to eliminate the asymmetric miscibility behavior.