In this paper we will discuss some phonetic differences that are found in the production of diphthongs. The study is of a limited scope and may be considered a pilot study. The kinds of questions that will be addressed have to do with the specification of timing in speech production. Are the vowel elements of diphthongs joined in language-specific ways, or can they be joined by more general principles, such as the assumption of a fairly constant transition duration, so that the transition rate will be faster if the distance between onset - and offset - vowels is greater? This kind of principle was proposed by Kent and Moll (1972) for some types of vowel-to-vowel sequences. Or does the duration of the transition lengthen as the distance of the transition increases, keeping the transition rate fairly constant? If any of these principles are at work, then their effects will show up acoustically in the transition duration, particularly of F2. If the Kent and Moll principle is applicable, it would for example operate as a tendency towards similar F2 transition durations in different languages for the ‘same’ diphthongs.