Using data from a corpus of spontaneous speech the present study investigates the timing and quality of diphthong components in Estonian, a quantity language that combines a rich vowel system (9 monophthongs, 36 diphthongs) with complex prosodic characteristics (three quantity degrees). In the Estonian quantity system, diphthongs are equivalent to long vowels and can occur in both long and overlong quantity degrees. Previous studies have been inconclusive as to whether the lengthening in the overlong quantity involves just one or both diphthong components. The results of the current study, based on the formant trajectories of 9 monophthongs and 8 diphthongs from 119 speakers, show that both diphthong components are lengthened but that the lengthening is more prominent in the second component. Instead of connecting two stable targets the formant trajectories change during the course of the whole diphthong forming a constant glide.