The journey of acquiring proficiency in a second and/or foreign language is lifelong for many learners. In the search for the best educational pedagogies, a study was conducted in which learners' motivations were categorised to analyse each driving effect over learning performance. A retrospective case study of 20 Chinese PhD students learning English in China (i.e. English as a Foreign Language--EFL) and in overseas (i.e. English as a Second Language--ESL) contexts was employed. This study identified 12 different motivational categories which were classified as controlled and autonomous motivations for learning English. Moreover, this study shows both continuity and change in students' motivations: there was a continuity of autonomous motivation over time and across the two contexts, whereas change (d = 0.54) was seen in the frequency of controlled motivation between the two contexts. Specifically, the motivation of these 20 students was dominated by passing exams or proficiency tests in the EFL context and by communication in the ESL context, which possibly accounts for 'silent English' phenomenon in China. Analysis of trends of changes in motivation across different study stages indicated that external regulation belonging to controlled motivation experienced the most fluctuation. This fluctuation was attributed to enrolment pressure in China as well as the immediate demand of communication and the high-stakes PhD study while studying abroad. The results suggest that the development of language learners' motivation is a dynamic process and contingent upon levels of schooling or learning contexts.