Talkers adopt different speech styles in response to factors such as the perceived needs of the interlocutor, environmental noise and explicit instruction. Some styles have been shown to be beneficial for listeners but many aspects of the relationship between speech modifications and intelligibility remain unclear, particularly for prosodic changes. The current study measures the relative intelligibility in noise of speech spoken in 5 speech styles – plain, infant-, computer- and foreigner-directed, and shouted – and relates listener scores to acous- tic/prosodic parameters and quantitative estimates of energetic masking. Intelligibility changes over plain speech correlated well with durational modifications, which included elongations of all segments as well as increases in the number of unfilled pauses. Both mean fundamental frequency and its range displayed great variation across styles but with no clear intelligibility benefits. Energetic masking per unit time was similar in each style but the total amount of speech which escaped masking was a good predictor of word identification rate. These findings suggest that much of the prosody-related intelligibility gain is derived from durational increases.