BACKGROUND: The present study examined, for the first time, the emotional burden of loneliness on dimensions of emotional valence and arousal, and its association with mental health outcomes. METHOD: A cross-sectional design was used, and data were collected from 503 adults across the UK with an online survey. Measures included socio-demographic characteristics, self-reported measures of loneliness and social isolation, affective ratings (i.e., valence and arousal) of loneliness experiences, and symptoms of depression and anxiety as mental health outcomes. RESULTS: The emotional burden of loneliness differed significantly across groups with differing loneliness experiences, and females scored significantly higher in the emotional burden of loneliness than males. The emotional burden of loneliness was associated with both depression and anxiety symptoms, and respectively added 4.7% and 6.2% of the variance, on top of measures of loneliness frequency and social isolation. CONCLUSIONS: Measuring the valence and arousal dimensions of loneliness experiences advances our understanding of loneliness experiences and its association with mental health outcomes. The theoretical, methodological, and practical implications of our study are discussed.