In English, nasal place assimilation occurs across word boundaries, such as ten bucks pronounced as te[m] bucks. Assimilation can be viewed as a reduction or loss of the assimilation target’s place cue (/n/ in ten), and simultaneously as an enhancement of the assimilation trigger’s place cue (/b/ in bucks) by spreading its place cue earlier in the signal. A message-oriented phonological approach predicts that assimilation is sensitive to the relative contextual inferrability of both the target and trigger words: More assimilation should be observed for more contextually predictable target words, while less assimilation should be observed for contextually more predictable trigger words. These predictions deviate from accounts that view assimilation solely as reduction. To test these predictions, sequences which license assimilation were extracted from a conversational speech corpus. Both categorical assimilation (based on close phonetic transcription) and gradient acoustic assimilation (based on F2) were analyzed. As predicted, assimilation was more likely both when a target like ten was high in predictability and when its trigger bucks was low in predictability. Assimilation thus serves as both reduction and enhancement, and can be used to manage redundancy in the speech signal. More broadly, this constitutes evidence for the influence of communicative pressures on phonology.