We have described a systematic study of block copolymers of the type AB in which one component is crystalline polyoxyethylene, following an observation in these laboratories that block copolymers adopt a regular structure in the presence of a preferential solvent for one block component of the system at appropriate concentrations. We have recently extended this study to the case of amorphous block copolymers of polyisoprene (block A) and polystyrene (block B) in the presence of a preferential solvent S for one or the other. The systems studied have been principally of the kind AB/S (two-block copolymers) and less thoroughly of the type BAB/S (three-block copolymers).