This article examines the role of distinctness between adjacent segments in consonant deletion. On the basis of five stop-deletion patterns, it establishes a correlation between the likelihood of cluster simplification and the level of similarity between the consonants in the cluster. This correlation is motivated on perceptual grounds, and an OT analysis of similarity avoidance is provided in which perceptual factors are integrated in the grammar through both faithfulness and markedness constraints. This perceptual approach improves in two ways on previous analyses, notably the OCP. First, it integrates similarity avoidance within a more general perception-based framework, which accounts naturally for its gradient nature. Second, it uncovers a distinction between absolute and contextual similarity avoidance between adjacent segments, depending on whether similarity avoidance is established without reference to the context in which the segments appear or relative to the quality of the perceptual cues available to the segments.