Light scattering measurements have been made on 10 polystyrene samples with weight average molecular weights M(w) of 1.2 x 10(3) to 3.1 x 10(6) in trans-decalin at temperatures T between 13 and 55 degrees C to determine the second and third virial coefficients (A(2) and A(3)) as functions of M(w) and T. The former coefficient at the Theta point (21 degrees C), where it vanishes for high M(w), is found to become negative and then sharply increase to positive with decreasing M(w). This molecular weight dependence of A(2) is similar to that recently observed for atactic poly(methyl methacrylate) in acetonitrile at the Theta temperature. The data of A(3) for high M(w) plotted against T have a broad positive minimum around the Theta point in contrast to the two-parameter theory prediction that A(3) vanishes at Theta and becomes negative below Theta, thus demonstrating the breakdown of the binary cluster approximation to A(3) Of polystyrene in trans-decalin near Theta. For low molecular weights, A(3) is insensitive to T and larger for lower M(w). The latter feature is ascribable to the effect from chain ends. From an analysis of the A(3) data for low M(w) at T between 17 and 35 degrees C the ternary cluster integral is estimated to be about 4 x 10(-45) cm(6), almost regardless of T. Its T-insensitive behavior bears some analogy with the temperature dependence of the third virial coefficient for imperfect monatomic gases near the Boyle point.