Email petitions to complete online surveys may be forwarded beyond the intended sample. We term this phenomenon the pass-along effect and investigate it as a factor that can influence the nature and size of survey samples in an online context. We establish the pass-along effect as a form of word-of-mouth communication and draw from the literature in this area to present and test a model of factors that influence the occurrence of this effect. The results of two studies provide empirical support for the existence and impact of the pass-along effect. Among the factors that lead to this effect are involvement and relationship with the survey topic, size of a participant's social network, and tie strength. The appropriateness of employing pass-along respondents as well as other implications for online sampling and survey research are discussed.