Previous research indicates that voice annotation helps reviewers to express the more complex and social aspects of a collaborative writing task. Little direct evidence exists, however, about the effect of voice annotations on the writers who must use such annotations. To test the effect, we desinged an interface intended to alleviate some of the problems associated with the voice modality and undertook a study with two goals: to compare the nature and quantity of voice and written comments, and to evaluate how writers responded to comments produced in each mode. Writers were paired with reviewers who made either written or spoken annotations from which the writers revised. The study provides direct evidence that the greater expressivity of the voice modality, which previous research suggested benefits reviewers, produces annotations that writers also find usable. Interactions of modality with the type of annotation suggest specific advantages of each mode for enhancing the processes of review and revision.