Sample members start web surveys but fail to complete them at relatively high rates compared to other modes. Existing theories and empirical findings on unit nonresponse may or may not apply to breakoff. This study contrasts breakoff and unit nonresponse in web surveys through response behavior for the same individuals across different surveys. Nonrespondents to one survey were considerably more likely to be nonrespondents to subsequent surveys, but such consistency in response behavior was substantially lower for breakoffs. There is a degree of transitioning between response behaviors, however, such as nonrespondents in one survey being more likely to be breakoffs than respondents in a subsequent survey, indicative of unmeasured common causes. Limited support for the common cause hypothesis is also found in demographic covariates, yet to a very limited degree; race and gender were associated with both breakoff and nonresponse, and some associations (e.g., year in school) were in the opposite direction. Subjects invited to multiple surveys in a short period of time were more likely than others to be nonrespondents, but were not more likely to break off