The second virial coefficient A(2) was determined from light scattering measurements for atactic oligo- and poly(alpha-methylstyrene)s (a-PalphaMS) with the fraction of racemic diads f(r) = 0.72 over a wide range of weight-average molecular weight M-w from 6.48 x 10(2) to 5.46 x 10(6) in cyclohexane at 30.5 degreesC (Theta) and in toluene at 25.0 degreesC, and for M-w greater than or equal to 2 x 10(5) in 4-tert-butyltoluene at 25.0 degreesC, in n-butyl chloride at 25.0 degreesC, and in cyclohexane at 35.0, 45.0, and 55.0 degreesC. It is shown that the observed characteristic features of A(2) at Theta, which is denoted by A(2,Theta), as a function of M-w in cyclohexane at Theta, i.e., a sharp increase with decreasing M-w for small M-w and a rather long-tailed negative region for large M-w, may be well explained by the theory that takes account of the effects of chain ends and also of three-segment interactions. It is also shown that the observed dependence of A(2) on M-w in toluene may be quantitatively explained by the theory that takes account of the effects of chain stiffness, local chain conformation, and chain ends. The results for the interpenetration function Psi appearing in A(2) (without the effects of chain ends) reconfirm that neither the two-parameter (TP) nor the quasi-TP theory can explain the behavior of Psi, as previously found for atactic polystyrene (a-PS) (f(r) = 0.59), atactic poly(methyl methacrylate) (a-PMMA) (f(r) = 0.79), and isotactic PMMA (f(r) similar or equal to 0.01); Psi as a function of the gyration-radius expansion factor depends separately on M-w and the reduced excluded-volume strength. A comparison is made of the present results for A(2,Theta) and Psi in toluene with previous ones for a-PS and a-PMMA in Theta and good solvents.