This report presents findings from an experimental study carried out in the context of the European Social Survey, to assess the impact a change in data collection mode from the current face-to-face interviewing to telephone might have on data quality and to study the likely causes of any observed mode effects. Evidence from previous studies suggests which differences in response can be expected between telephone and face-to-face interviewing and also suggests likely causes of such differences. Previous empirical studies are, however, often limited in their ability to isolate different causes. The experimental design enabled us to distinguish mode effects caused by differences in the type of question stimulus used in each mode (audio vs. visual) and mode effects caused by the presence or absence of the interviewer. The design included three comparison groups (two interviewed face-to-face (one with showcards, one without) and the third by telephone). We found evidence of effects caused by the presence of the interviewer, but few stimulus effects. We tested a number of hypotheses about the likely causes of mode effects on response, focusing on three forms of satisficing and social desirability bias. We found no evidence that using showcards influenced response quality, either positively or negatively. Unlike previous studies, we found no support for the hypothesis that telephone respondents were more likely to satisfice. However, consistent with our expectations, we did find telephone respondents were more likely to give socially desirable responses across a range of indicators.