This article reports a study of the effect of composition in styrene-methyl methacrylate random copolymers on K and alpha constants in the Mark-Houwink equation. Copolymers with a variety of compositions and chain lengths were prepared through a controlled free-radical copolymerization, with benzyl diethyldithiocarbamate and tetraethyl thiuram disulfide as iniferters. The synthesized products were analyzed with several techniques, including Fourier transform infrared, H-1 NMR, gel permeation chromatography, and viscometry. By relating the determined constants K and alpha of various copolymers to their composition, we found that the constant varied nonlinearly with the composition. The constant alpha decreased with increasing poly(methyl methacrylate) (PMMA) content in the copolymer molecule up to 60 wt %. After that, the constant increased with the PMMA content, reaching the value of the PMMA homopolymer, However, the constant K initially increased with the PMMA content up to a critical composition (60 wt %) and subsequently decreased with further increasing PMMA content. These results suggest that the molecular weight of a polystyrene-PMMA random copolymer of known composition cannot be approximated with a simple linear equation comprising the K and alpha values of each relevant homopolymers. The aforementioned trends are qualitatively discussed in relation to some possible sequential distribution in the copolymer molecules and the resulting conformation.