The present study challenges the prevalent negative view of personal smartphone use at work and advances our understanding of its effects on the work–nonwork interface. Drawing on border theory, we focus on the social and instrumental feature of smartphones that allows employees to be connected with their private contacts (e.g. friends and family members) while at work and, thereby, test the influence of day-level personal smartphone use at work on positive and negative work–nonwork interaction (WNI). In considering social experiences of smartphone-enabled communication, we propose underlying explanatory mechanisms, namely, the social benefits (i.e. feeling in touch with private contacts) and social costs (i.e. feeling expected to respond to private contacts) of communication. One hundred and twenty-seven employees participated in a diary study. They completed online questionnaires three times a day over ten working days, resulting in a total of 1,066 days of data. Multilevel structural equation modelling indicated that on the day-level of analysis, personal smartphone use at work promoted positive WNI through the perceived social benefits of communication and increased negative WNI through the perceived social costs of communication. We discuss our findings and extend current research on personal smartphone use at work by acknowledging this widespread workplace behaviour as both beneficial and detrimental for successfully managing the daily interplay of the work and nonwork domains.