The poly(amic ethyl esters) (PAEs) of pyromellitic dianhydride/4,4/-oxydianiline (PMDA/ODA) were prepared in three isomeric forms, with meta-linkages, para-linkages and randomly alternating meta-and para-linkages (referred to as mixed PAEs). In addition to the normal PAEs (which contained only hydrogen and which were cast as base layers for diffusion studies), each of these isomeric PAE was synthesized in two other forms : (1) labelled with deuterium on the ODA segments, and (2) labelled with deuterium on the ethyl ester moiety to allow determination of the imide fraction, f. Forward recoil spectrometry (FRES) was utilized to provide information about the polyimide-polyimide interface by measuring the depth profile of hydrogen and deuterium for bilayer samples consisting of a thick non-deuterated base layer, and a thin deuterated tracer layer. FRES measurements indicated that the interdiffusion distance, w, decreased markedly with increasing imidization temperature, T-i. For base layers prepared from N-methylpyrrolidone, w for the para-and mixed PAE base layers decreased more rapidly with T-i than with w for the meta-PAE base layers. However, these differences were found to result from the differences in imidization kinetics of the various isomers with I increasing more rapidly with T-i for para-and mixed PAE than for the meta-PAE. Nevertheless, when w was plotted as a function of f, all of the isomers exhibited similar behaviour, i.e. w decreased steadily as the imide fraction f approached 1. Base layers cast from dimethyl sulfoxide did not show differences in either f or w after imidization at a given T-i.