Critical lines and the phase behavior of a compressible polymer-solvent system are investigated using of the full Tompa model for varying numbers of segments in each polymer chain. Critical lines are displayed in the density-density and the combined P,T and T,x planes for the "right corner" and also "left corner" cases. The crossing point, in a point at which two critical lines meet and exchange branches, is also calculated analytically. The P,T diagrams are discussed using the Scott and van Konynenburg phase diagram classifications. It is found that the right corner case corresponds to the transition from type II to type III classifications. On the other hand, the left corner case does not correspond to any of the six main phase diagram classes. (C) 1998 American Institute of Physics.