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This study investigated the relationship between tongue movements during the production of German diphthongs and their acoustic time intervals. To this end, five subjects produced a set of logatomes that contained German primary, secondary, and peripheral diphthongs in the context of bilabial and labiodental consonants at three different speaking rates. During the utterances, tongue movements were measured by means of optical palatography (OPG), i.e. by optical distance sensing in the oral cavity, along with the acoustic speech signal. The analysis of the movement signals revealed that the diphthongs have s-shaped tongue trajectories that strongly resemble half cosine periods. In addition, acoustic and articulatory diphthong durations have a linear, but not proportional, relationship. Finally, the peak velocity and midpoint between the two targets of a diphthong are reached in the middle of both the acoustic and articulatory diphthong time intervals, regardless of the duration and type of diphthong. These results can help to model realistic tongue movements for diphthongs in articulatory speech synthesis.