Motivation to learn a second language (L2) is considered a crucial individual factor in explaining success or lack thereof in second language learning. This study examined learners' motivation to learn Chinese as a second language (L2). The study was longitudinal and cross sectional. Interviews were conducted with learners at different year levels over the course of a university semester to analyse motivation to choose a particular L2, how motivation changes over time, and factors contributing to that change. The study found that motivation is diverse, complex, and undergoes many fluctuations. Motivation to begin language study was related to both past L2 learning experiences and personal goals. Once language study began, factors related to the learning environment were the most important factors to impact on motivation, and were also most likely to demotivate learners. However, when learners had a clear future image of themselves as speakers of the L2, they were able to continue motivating themselves, regardless of the demotivation stemming from their learning environment. These findings lend some support for Dornyei's (2005, 2009) L2 Motivational Self-System theory, which highlights the importance of the ideal L2 self as a powerful motivator to learn an L2. (Contains 1 endnote and 8 tables.)