We have studied phase boundaries and critical scattering on equivalent systems of homopolymer blends and diblock copolymers with different end groups terminating the polymer chains. The aim is to investigate the effect of end groups relative to the effect of the covalent junction in diblock copolymers. Either effect intuitively causes corrections with a V-1 dependence on the molar volume V. Besides the experimental phase boundaries, the Flory-Huggins interaction parameter has been determined and is discussed. The polymer systems studied is based on poly(styrene), PS, and poly(ethylene oxide), PEO, where the latter polymer chain is terminated by either a hydroxy or a methoxy group. The change from hydroxy to methoxy end groups caused a change in the phase boundary of 90K and 45K for homopolymer blends ((V) over bar = 1970 cm(3)/mol) and diblock copolymers ((V) over bar = 17 700 cm(3)/mol), respectively.