Purpose: The study aims to give a descriptive account of university students' engagement with non-learning-related activities during class time and explore the relationship between off-task multitasking and learning. The predictive factors for off-task multitasking from individual, social and class-related dimensions are also examined. Design/methodology/approach: Contextualized in a comprehensive university in Hong Kong, the study adopts a survey design and involves 79 samples. Findings: The data show that Hong Kong university students are avid users of mobile phones and heavily engage with digital devices. Off-task multitasking with mobile phones is a common phenomenon, yet not related to learning performance. Among the various media and apps on mobile phones, instant messenger stands out as the most frequently used app on a daily basis and inside the classroom. The individual device-use habit and classroom engagement are significant predictors for off-task multitasking during class time. Practical implications: This paper will allow teachers and students to be more aware of the causes and effects of off-task multitasking behaviors during class time and derive practical guidance and strategies to pay heed to and resist the disruptive influence of technologies on learning. Originality/value: The existing scholarly work show a mixed and incomplete picture regarding the effects and determining factors of students' multitasking. This study includes three variables from individual, social and teaching/learning dimensions and seeks to evaluate their predictive strengths. The results of the study will deepen our understanding of the patterns of off-task multitasking.