There is growing evidence that Internet-mediated psychological tests can have satisfactory psychometric properties and can measure the same constructs as traditional versions. However, equivalence cannot be taken for granted. The prospective memory questionnaire (PMQ; Hannon, Adams, Harrington, Fries-Dias, & Gibson, 1995) was used in an on-line study exploring links between drug use and memory (Rodgers et al., 2003). The PMQ has four factor-analytically derived subscales. In a large (N763) sample tested via the Internet, only two factors could be recovered; the other two subscales were essentially meaningless. This demonstration of nonequivalence underlines the importance of on-line test validation. Without examination of its psychometric properties, one cannot be sure that a test administered via the Internet actually measures the intended construct.