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 (T-m degrees) 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 alpha- or beta-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 (l(c)) and temperature were identified; the corresponding T-m degrees value was then determined via constructing the Gibbs-Thomson melting line and extrapolating it to the infinite crystal thickness. Generally, the T-m degrees of alpha and beta crystals decreases with increasing comonomer content, and the T-m degrees for the beta phase was always higher than that of the a counterpart. The level of T-m degrees depression was interpreted in terms of the Sanchez-Eby theory, showing that the introduction of methyl groups into the beta structure has higher penalty energy than that into the a structure in both sPS-3MS and sPS-4MS cases, suggesting that incorporating the methyl groups more significantly destabilizes the beta crystals and leads to a further preference for a crystal formation. Comparing the beta crystals between sPS-3MS and sPS-4MS systems, the penalty energy for incorporating 4MS units into the beta lattice is greater than that in the 3MS case; for the a crystals, the penalty energy for incorporating 3MS units is greater than that in the 4MS case.