The popularity of mobile devices, and particularly smart phones, has been driven by rapid developments in their power, capabilities and features. However, these developments create new and additional demands on users. Increased attention and more complex thought patterns are required to interact with such technology. Testing three popular smart phones, this research applies the principles of Cognitive Load Theory to evaluate the impact of varied operating systems and interaction modes on users' cognitive load. NASA Task Load Index was used to measure participants' perceptions of the cognitive load imposed by interaction with these mobile devices. While different operating systems were found to have a significant impact on cognitive load, interaction modes (sitting or walking) had little impact on users' perceptions of cognitive demands of engagement. Mobile device application designers may conclude that field-based experiments are therefore avoidable, allowing development effort to be focused on improving other aspects of interaction.