Non-work-related computing (NWRC) refers to any voluntary act of employees using their companies’ Internet access during office hours for non-work purposes. This study attempts to investigate the causes of NWRC. The model was developed based on rational decision-making and tested through a survey of 167 employees in China and Singapore. Results reveal that NWRC intention is a significant predictor of NWRC behaviour, and NWRC attitude and subjective norms are significant predictors of behavioural intention as in many prior studies. Besides, the perceived benefits and costs affect NWRC attitude. However, it turns out that habit may overrule other rational decisional factors when habit is included in the model. Finally, the comparison between two groups with different levels of control mechanisms shows the moderating effect.