The findings discussed in this paper are part of a broader fieldwork study where the characteristics of infant-directed singing in natural interaction are analyzed. The aim of this part of the study was to determine whether vowels in infant-directed singing were of a clearer speech quality compared to adult-directed speech as it has been shown several times for ID speech. Six German speaking mothers sang for their children aged between 2 and 10 months. Stressed long vowels /a:/, /i:/, /u:/ of these songs were analyzed and compared to vowels in samples of AD speech. Results show that mothers use acoustically more extreme vowels when singing for their infants than in AD speech. This indicates that singing forms a subpart of the infant-directed register which is well-adapted to assist the child at an early stage in acquiring the linguistically relevant characteristics of his/her mother tongue.