This paper presents the results of a study concerning the acoustic correlates of contrastive word stress in bisyllabic and trisyllabic German words, produced by four children aged 2;3 to 7;3 and their mothers. We found that German children of that age are certainly able to produce contrastive word stress and that vowel duration is the most reliable correlate of word stress in the utterances produced by all four children and their mothers, independent of the position of the vowel within the produced word. Furthermore, we found the voice quality parameter incompleteness of closure most uniformly used by the mothers to mark word stress while the children are on different acquisition stages for this parameter.