This mixed-method study aims to investigate science students' motivational orientations and their self-perceived experiences contributing to self-determination in EFL reading at a secondary school in Thailand. Drawing upon the questionnaire and semi-structured interviews, the findings revealed that the overall science students' EFL reading motivations were moderate. Each of their motivational orientations was ranked as follows: identified regulation (ID), intrinsic motivation (IM), external regulation (ER), introjected regulation (IJ), and amotivation (AM), accordingly. ID, a motivational orientation with regard to autonomous motivation, seemed to best correspond with the science students' perceptions. They also valued the influence of social-contextual variables (i.e., families, peers, and teachers) as relatively important to their behavioural control toward EFL reading (ER and IJ). This study offers pedagogical implications to improve science students' EFL reading motivation.