The aims of this study were (1) to examine relations between baseline levels of loneliness and momentary affect and perceptions of company, and (2) to test responses to perceived social threat and lowered reward response to positive stimuli in relation to loneliness in adolescents. Data were collected among 278 adolescents (Mage = 14.19, 59% girls) by using the experience sampling method. Baseline loneliness was related to affect and appraisals of company. Findings revealed greater responses to social threat, in that adolescents with higher levels of baseline loneliness were more negatively affected by negative company. For the lowered reward response, we found opposite effects: adolescents high on loneliness were more positively affected by positive company.