Speakers reuse prior references to objects when choosing reference phrases, a phenomenon known as lexical entrainment. One explanation is that speakers want to maintain a set of previously established referential precedents. Speakers may also contrast any new referents against this previously established set, thereby avoiding applying the same reference phrase to refer to different referents, a complementary phenomenon I call lexical differentiation. This study provides evidence for lexical differentiation in the context of lexical entrainment. Both phenomena are present when speakers and addressees interact, when speakers imagine addressees, and when speakers simply name objects. This indicates that lexical entrainment and lexical differentiation may be products of speaker-centered processes. However, the magnitudes of these effects differ when speakers have different audience demands, indicating that audience-centered processes may also be involved. (Contains 6 figures and 8 tables.)