This study aims to compare the effects of incentives and contact modes on survey completion rates, and examine how non-response rates might affect the quality of responses in online surveys. Two online surveys with different incentives and contact modes were conducted. In study 1, both pre-paid cash incentives and a sweepstakes offering of equal monetary value were more effective than no incentive but the pre-paid incentive were more effective than the sweepstakes. In study 2, respondents who received a pre-paid cash incentive via postal mail after initially failing to respond to an emailed sweepstakes offer differed from the initial respondents with respect to demographic and psychological characteristics previously found to be important in a widely cited study of social networking. Therefore, non-response error may indeed be a problem in online surveys, at least among those directed to college students and using a common method of recruitment in which offers of sweepstakes drawings are delivered via email. Not only do the levels of variables important to an understanding of online communication vary by survey method, but also the lawful relationships among variables differ between groups recruited by different methods.