This paper investigates the effects on response and breakoff rates in Web surveys of e-mail invitation design elements including: subject line, estimated survey duration, and invitation length. The analysis is focused not only on the main effects, but also on an issue which has not yet been studied systematically: interaction effects between these design elements. A factorial complete block design Web experiment – among students, faculty, and administrative staff at the Higher School of Economics in Russia – varying the estimated length of the survey (10 vs. 20 minutes), subject line (formal “monitoring” vs. informal “help” request), and invitation length (short vs. long) was conducted. We discuss the results of the experiment and argue that we should not ignore interaction effects between design elements to understand response in Web surveys more thoroughly.