Three studies explored how vowel sounds are utilised by mothers and infants in the first year to regulate emotional expression. In Study 1, a cultural comparison was carried out. 6 English-speaking and 6 Japanese-speaking mother-infant dyads were filmed in their homes (3 male and 3 female infants in each country), when the infants were aged 4 months. Analysis was carried out of vowel sounds produced by mothers and infants and of bodily contact in 2 defined emotional situations (‘engagement’ and ‘disengagement’). The findings presented here suggest that acoustic features of vowel sounds (pitch, intensity and duration) were found to be coordinated with bodily contact and correlated with specific emotional communicative contexts. Study 2 and 3 were only carried out in Scotland with English-speaking participants. In Study 2, an ‘emotional voice’ experiment was developed to test 11 infant’s reactions to changes in pitch variation in mother vowel sounds, in specific emotional situations. In Study 3, 158 adult participants were asked to judge if isolated infant vowel-like calls (which had previously been coded for emotional content) expressed distinct emotions, and whether they felt any emotional response to the sounds. The findings suggest that acoustic features of vowel sounds appear to be utilised differently in ‘engaged’ and ‘disengaged’ interactions and in Japan and Scotland, suggesting that emotional communication develops in culturally specific ways. Results suggest that adults may be able to recognise emotional meanings in infants’ early vowel sounds, and that infants often react in distinctive ways to emotional changes in their mothers’ voices.