Energy Conservation Theory of Second Language Acquisition (ECT-L2A) (Han, Bao, & Wiita, 2017b), an interdisciplinary theory juxtaposing a physics perspective and an applied linguistics perspective, conceptualizes individuals' ultimate attainment as a function of dynamic transformation of endogenous and exogenous energies throughout the learning process. ECTL2A predicts, "inter alia," (1) that beginning L2 learners should show stronger motivation than end-state learners, (2) that developing learners should show higher motivation than that of beginners, and (3) that beginning, developing, and end-state learners should exhibit similar profiles in aptitude. These predictions were tested with 56 ESL learners. Correlation analyses were conducted on learners' aptitude, motivation, years of study, and performance on a timed grammaticality judgment test. The results showed that: (1) with increase in proficiency and years of study, there was a decrease in motivation; (2) the effect of aptitude diminished as learners made progress towards the target language; (3) combined effect of aptitude and motivation correlated positively with the their L2 attainment, but its effects diminished as the proficiency level of the groups became more advanced.