The human brain is continuously confronted with dynamic visual input, and from this it must infer whether input belongs to a single versus multiple object identities across time. Object substitution masking (OSM), in which perception of a target stimulus is impaired by a temporally trailing 4-dot mask, reflects a failure to segment the target and mask as discrete objects. According to Bouvier and Treisman (2010), OSM only occurs for targets that require binding multiple separate features (e.g., color and orientation) to be identified. In contrast, a target that represents a unique feature is thought to be impervious to masking. Here, however, we show that a single orientation target (a Gabor) is susceptible to masking with an orientation-discrimination task, but only when the mask is similar in orientation to the target. That is, target-mask similarity, rather than target complexity determines masking. A reexamination of Bouvier and Treisman's (2010) results show that they can be explained within this target-mask similarity perspective. This means that the similarity of 2 objects determines whether they will be integrated or segmented across time, rather than the complexity of 1 of the objects in isolation.