German primary diphthongs are conventionally transcribed using the same symbols used for some monophthong vowels. However, if the corresponding vocal tract shapes are used for articulatory synthesis, the results often sound unnatural. Furthermore, there is no clear consensus in the literature if diphthongs have monopthong constituents and if so, which ones. This study therefore analyzed a set of audio recordings from the reference speaker of the state-of-the-art articulatory synthesizer VocalTractLab to identify likely candidates for the monophthong constituents of the German primary diphthongs. We then evaluated these candidates in a listening experiment with naive listeners to determine a naturalness ranking of these candidates and specialized diphthong shapes. The results showed that the German primary diphthongs can indeed be synthesized with no significant loss in naturalness by replacing the specialized diphthong shapes for the initial and final segments by shapes also used for monopthong vowels.