Infants prefer to listen to infant-directed speech (IDS) over adult-directed speech (ADS). IDS contains a greater amount of affect than ADS (Singh, Morgan & Best, 2002). Affect in infant-directed speech has been said to foster social bonds, maintain attention and teach language. In order to identify phonetic correlates of affect, prosodic features such as fundamental frequency, pitch range, pitch contour, vowel duration and rhythm have been tried (Katz, Cohn & Moore, 1996; Trainor, Austin & Desjardins, 2000). However, affect ratings are typically carried out on low-pass filtered speech in order to obscure semantic cues to affect. It is possible that more than semantic meaning is distorted by the filtering process. In the present study, acoustic-phonetic correlates to affect were studied in un-filtered short speech segments. One-syllable speech segments were rated on a scale ranging from highly negative via neutral to highly positive affect. Formant (F1, F2, F3), pitch (mean, maximum, minimum, range, contour), and vowel duration measures were obtained from the speech samples, and relations between acoustic measures and rated affect were analyzed. The speech samples were the syllables /mo/, /na/, and /li/ produced by Swedish mothers (n = 29) and fathers (n = 21) when talking to their 12-month-old children. Recordings of IDS took place during free play in a laboratory setting, and the syllables were the names of soft toys that the parents were asked to use when interacting with their child. Parents and children participated in a longitudinal interaction study, and this was their fourth visit at the laboratory, so they were familiar with task, setting and toys. ADS exemplars of the syllables were also selected from a sub-sample of the mothers (n = 14), recorded at their first visit to the laboratory. Participants in the perceptual rating experiment (n = 35; 21 female; mean age = 28.6 years; age range = 19-45 years) were presented with one syllable at a time and asked to rate the affect conveyed on a scale from -4 (high negative affect) to +4 (high positive affect), with 0 as midpoint (neutral affect). The experiment was self-paced, and participants could listen to each syllable as many times as they liked. Each experiment session lasted between 30 and 50 minutes. A mixed-effects model was designed with AffectRating as dependent variable, Rater as random effects variable, and RaterGender, RaterHasChildren, F1, F2, F3, MeanPitch, PitchRange as well as VowelDuration as fixed effects variables. Minimum pitch, maximum pitch and pitch contour were excluded from the analysis since they were correlated with pitch range. Significant results were found for F1, F3, MeanPitch, PitchRange and VowelDuration. Higher F1 and/or F3 resulted in more negative perceived affect whereas higher mean pitch, greater pitch range, and/or longer vowel duration resulted in more positive perceived affect. The relation between perceived affect and formant values could be related to differences in perceived affect for different vowels, rather than variations in the formant values per se. It would be interesting to look at variation within separate vowel categories. The relation between positive affect and prosodic exaggerations suggests that some acoustic characteristics of IDS could be a result of parents conveying positive affect to their children.