After randomly incorporating 2–21 mol % of 3-methylstyrene (3MS) or 4-methylstyrene (4MS) into the syndiotactic polystyrene (sPS) backbone, the effects of the comonomer units on the equilibrium melting temperature (Tm°) depression were examined within the Sanchez–Eby theoretical framework. In situ small-/wide-angle X-ray scattering (SAXS/WAXS) was used to investigate the morphological evolution upon heating. The WAXS profiles showed that the α- or β-dominated crystals in syndiotactic poly(styrene-stat-3-methylstyrene) (sPS-3MS) and poly(styrene-stat-4-methylstyrene) (sPS-4MS) all consistently reach complete melting with no signs of interphase transformation. By analyzing the corresponding SAXS heating profiles with a polyradius cylinder form factor, the correlations between the crystal thickness (lc) and temperature were identified; the corresponding Tm°value was then determined via constructing the Gibbs–Thomson melting line and extrapolating it to the infinite crystal thickness. Generally, the Tm°of α and β crystals decreases with increasing comonomer content, and the Tm°for the β phase was always higher than that of the α counterpart. The level of Tm°depression was interpreted in terms of the Sanchez–Eby theory, showing that the introduction of methyl groups into the β structure has higher penalty energy than that into the α structure in both sPS-3MS and sPS-4MS cases, suggesting that incorporating the methyl groups more significantly destabilizes the β crystals and leads to a further preference for α crystal formation. Comparing the β crystals between sPS-3MS and sPS-4MS systems, the penalty energy for incorporating 4MS units into the β lattice is greater than that in the 3MS case; for the α crystals, the penalty energy for incorporating 3MS units is greater than that in the 4MS case.