For decades, researchers and the general public have debated whether children without siblings differ from children with siblings in ways that are meaningful for development. One area that is underexplored in the literature on only children versus children with siblings concerns time use and emotional states in alone time and in social interactions. Resource dilution theory and the prior literature suggests that adolescent only children and adolescents with siblings may differ in some social interactions, such as in time with parents, but not in others, such as in time alone, due to offsetting effects or the universality of certain experiences among adolescents. This study tested these arguments by comparing companionship patterns and four emotional states (happiness, sadness, stress, and meaningfulness) among adolescents (ages 15-18) without siblings (N= 465) and adolescents with siblings (N= 2513) in the nationally representative American Time Use Survey (2003-2017). Relative to adolescents with siblings, adolescents without siblings spent more time alone, similar amounts of time with peers, and more time exclusively with parents. Only children were not as happy when spending time alone and with peers as adolescents with siblings, but their emotions in these settings were not more negative or less meaningful. In most other social interactions, emotional states were similar between adolescents with and without siblings. These findings show that adolescents with and without siblings differed mainly in their companionship patterns within the household and in their levels of happiness when alone and with peers.