More and more respondents use mobile devices to complete web surveys. These devices have different characteristics, if compared to PCs (e.g. smaller screen sizes and higher portability). These characteristics can affect the survey responses, mostly when a questionnaire includes sensitive questions. This topic was already studied by Mavletova and Couper (2013), through a two-wave experiment comparing PCs and mobile devices results for the same respondents in a Russian opt-in panel. We replicated this cross-over design, focusing on an opt-in panel for Spain, involving 1,800 panellists and comparing PCs and smartphones. Our results support most of Mavletova and Couper’s (2013) findings (e.g. generally the used device does not significantly affect the reporting of sensitive information), confirming their robustness over the two studied countries. For other results (e.g. trust in data confidentiality), we found differences that can be justified by the diverse context/culture or by the quick changes that are still characterizing the mobile web survey participation.