There are different ways of recruiting participants for internet-mediated research. Small differences in personality have previously been documented between participants recruited in different ways. Three online studies investigated whether such personality biases could affect research outcomes. In Study 1, volunteers completing an online personality test scored higher on Openness to Experience than students participating as a course requirement. Study 2 demonstrated that Openness to Experience was associated with political voting preference (Republican v. Democrat). Study 3 found that volunteers scored higher on Openness to Experience than members of a paid research panel. Among volunteers, but not paid panellists, Openness to Experience was associated with voting preference. It is concluded that where research outcomes may be influenced by personality, particularly Openness to Experience, researchers conducting online studies should be aware that biases arising from recruitment techniques could influence findings. Recommendations are made for addressing this issue.