Control-value theory suggests that students' control and value appraisals mediate the relation between contextual supports and student emotions in formal learning settings; however, this theory has not been tested in informal learning contexts. Understanding mechanisms for instructional support in informal learning contexts can inform the design of effective instruction both in and out of school. In this study, we tested control-value theory by examining whether youths' momentary appraisals of control and value for activities mediate the relation between program quality indicators and state emotions in summer STEM programs focused on science, technology, engineering, and mathematics for middle- and high-school youth. Participants were 203 youth aged 10-16 years attending nine summer STEM programs in the US. Youth ranged in age from 10 to 16. Data were collected via the experience sampling method and video recordings. Trained coders reviewed video footage and coded for program quality. Structural equation modeling demonstrated that most program quality variables explored were not predictive of youths' appraisals of control and value, but state emotions varied based on program quality. Youth reported lower boredom when active participation and higher-order thinking were rated as high by trained observers. Youth also reported high excitement when activities involved high levels of higher-order thinking. High appraisals of control were associated with high levels of happiness and excitement and low levels of frustration, whereas high appraisals of value predicted high levels of frustration. Theoretical and practical implications are discussed.