In order to evaluate the quality of the "building blocks" in a diphone-based rule synthesis system for the French language, a listening test was designed to measure the intelligibility of all diphones. Several sets of nonsense words of the type CVVC and VCCV (C=consonant, V = vowel) were presented to 8 native French listeners for identification. As a reference, a subset of LPC-resynthesized words was presented as well. A PC-based listening facility was developed for efficient testing. The results of this intelligibility experiment allow for diagnostic evaluation.