This article explores how mobile phones function as an affective technology for young adults, by adapting the self-expansion model to understand attachment to mobile phones. In an online survey, 272 smart phone users reported on their recalled responses to loss/separation from their mobile phone (not by choice), as well as their use of the mobile phone for self-expansion. Results show that self-expansion via mobile phone was associated with greater inclusion of the mobile phone in the self-concept and greater subjective well-being. Most respondents reported negative feelings, such as loneliness/disconnection, anxiety, and boredom, when without their mobile phone, but others felt relieved to be out of touch with others. The use of the mobile phone for self-expansion was associated with more negative emotion and less positive emotion (relief) in response to loss/separation from the phone. Interpretations of the findings are discussed.