In this paper we are presenting structural investigations on the phase separation behaviour of a novel high temperature stable polymer blend of a polysulfone (PSU) and a perfectly amorphous polyamide (PA). The phase separation kinetics of thin films cast onto glass slides and of bulk samples was observed by means of various microscopic methods such as optical, laser scanning confocal, scanning electron and transmission electron microscopy. The phase separation process was found to be suppressed by adding a small amount of compatibilizers such as chemically modified PSU, i.e. introducing a small amount of carboxylic groups, which causes the formation of covalent bonds with the PA at the interface of the PSU and PA or by adding a third component, phenoxy, which is supposed to be mainly located at the interface of the PSU and PA and to freeze the phase separation due to its attraction to both the PSU and PA. (C) 1997 Elsevier Science Ltd.