This paper investigates the factors that influence 15-year-old students' intentions to study physics post-16, when it is no longer compulsory. The analysis is based on the year 10 (age, 15 years) responses of 5,034 students from 137 England schools as learners of physics during the academic year 2008-2009. Factor analyses uncovered a range of physics-specific constructs, 7 of which were statistically significantly associated with intention to study physics post-16 in our final multi-level model; in descending order of effect size, these are extrinsic material gain motivation, intrinsic value of physics, home support for achievement in physics, emotional response to physics lessons, perceptions of physics lessons, physics self-concept and advice-pressure to study physics. A further analysis using individual items from the survey rather than constructs (aggregates of items) supported the finding that extrinsic motivation in physics was the most important factor associated with intended participation. In addition, this item-level analysis indicated that, within the advice-pressure to study physics construct, the encouragement individual students receive from their teachers is the key factor that encourages them to intend to continue with physics post-16.