There is a long-standing debate in the literature about the benefits that acoustic components of Infant Directed Speech (IDS) might have for infants' language acquisition. One of the highly contested features is vowel space expansion, which refers to the enlargement of the acoustic space between the corner vowels /i, u, a/ in IDS compared to Adult Directed Speech (ADS). Some evidence indicates that vowel space expansion in IDS facilitates infants' speech perception, thus promoting language development, whereas other studies have questioned these benefits and have proposed that any processing benefits of IDS are due to its other prosodic features such as exaggerated and variable pitch. This study aimed to tease apart the effects of vowel space expansion and prosodic exaggeration in IDS on 18-month-old infants' speech processing. Using a looking-while-listening paradigm, two between-subjects conditions were compared: Exaggerated Pitch (with exaggerated pitch height and range, but without vowel space expansion) and Expanded Vowel Space (with vowel space expansion, but no exaggeration in pitch height and range). Our results showed that infants recognized the meanings of the words more accurately in the Expanded Vowel Space compared to the Exaggerated Pitch condition. This suggests that vowel space expansion in IDS facilitates infants' lexical processing even when it does not cooccur with the prosodic exaggeration typical of IDS.