Participation in web surveys via smartphones increased continuously in recent years. The reasons for this increase are a growing proportion of smartphone owners and an increase in mobile Internet access. However, research has shown that smartphone respondents are frequently distracted and/or multitasking, which might affect completion and response behavior in a negative way. We propose 'SurveyMotion (SMotion)', a JavaScript-based tool for mobile devices that can gather information about respondents' motions during web survey completion by using sensor data. Specifically, we collect data about the total acceleration (TA) of smartphones. We conducted a lab experiment and varied the form of survey completion (e.g. standing or walking). Furthermore, we employed questions with different response formats (e.g. radio buttons and sliders) and measured response times. The results reveal that SMotion detects higher TAs of smartphones for respondents with comparatively higher motion levels. In addition, respondents' motion level affects response times and the quality of responses given. The SMotion tool promotes the exploration of how respondents complete mobile web surveys and could be employed to understand how future mobile web surveys are completed.