The second virial coefficient A2 was determined for atactic oligo- and polystyrenes in methyl acetate below, at, and above ϑ (41.5 °C) in the range of weight-average molecular weight Mw from 4.74 × 102 (tetramer) to 7.72 × 106. The gyration-radius expansion factor αS was also determined for the sample with Mw = 7.72 × 106. It is found that A2 depends on Mw appreciably in the range of small Mw. Although the dependence of A2 on Mw in methyl acetate is different from that previously found in cyclohexane, the former may also be explained quantitatively by the Yamakawa theory that takes account of the effect of chain ends, indicating that the difference in the Mw dependence between A2 in the two ϑ solvents arises from that between the effects of chain ends. An analysis gives values of the effective excess binary-cluster integrals β1 and β2 associated with the chain end beads and also the binary-cluster integral β between intermediate identical beads as functions of temperature T, all of them except for β above ϑ being quadratic in τ = 1 − ϑ/T. With these values of β, the conventional and scaled excluded-volume parameters z and z̃ below ϑ are calculated. The results for the part of A2 without the effect of chain ends along with those previously determined in cyclohexane give a single-composite curve below ϑ when Mw1/2 is plotted against z, being consistent with the two-parameter theory prediction irrespective of the difference in solvent condition. This is in contrast to the behavior of above ϑ. It is also found that if αS is plotted against z̃ (or z for large Mw), the present data points along with those in cyclohexane form a single-composite curve, indicating that the quasi-two-parameter theory is valid for αS below ϑ as well as above ϑ irrespective of the difference in solvent condition.