Due to methodological problems, the quality of the outcomes of web surveys may be seriously affected. This paper addresses one of these problems: self-selection of respondents. Self-selection leads to a lack of representativity and thus to biased estimates. It is shown that the bias of estimators in self-selection surveys can be much larger than in surveys based on traditional probability samples. It is explored whether some correction techniques (adjustment weighting and use of reference surveys) can improve the quality of the outcomes. It turns out that there is no guarantee for success.