Children’s polysyllables were investigated for changes in (1) consonant and vowel accuracy, (2) error frequency and (3) polysyllable maturity over time. Participants were 80 children (4;0-5;4) with phonologically-based speech sound disorders who participated in the Sound Start Study and completed the Polysyllable Preschool Test (Baker, 2013) three times. Polysyllable errors were categorised using the Word-level Analysis of Polysyllables (WAP, Masso, 2016a) and the Framework of Polysyllable Maturity (Framework, Masso, 2016b), which represents five maturity levels (Levels A-E). Participants demonstrated increased polysyllable accuracy over time as measured by consonant and vowel accuracy, and error frequency. Children in Level A, the lowest level of maturity, had frequent deletion errors, alterations of phonotactics and alterations of timing. Participants in Level B were 8.62 times more likely to improve than children in Level A at Time 1. Children who present with frequent deletion errors may be less likely to improve their polysyllable accuracy.