This study investigates the motivation and demotivation of adult online learners studying English as a foreign language, using D & ouml;rnyei's (2005, 2009) L2 Motivational Self-System as the main framework. A survey of 151 participants from a Korean cyber university included items on L2 motivation, demotivation, motivated learning behavior, and English achievement. Factor analysis identified five motivational factors: Ideal L2 Self, Ought-to L2 Self, Cultural Interest, Instrumentality-Job, and Instrumentality-Altruism, and three demotivational factors: Self-regulation, Learning Content, and Online Learning. Repeated measures ANOVA analyses revealed that Cultural Interest and Ideal L2 Self were the dominant motivational factors, while Online Learning was the least significant demotivator. Correlation analysis showed the Ought-to L2 Self was significantly related to all demotivators. Regression analysis showed that demotivators related to online learning, rather than motivational factors, were the strongest predictors for both English achievement and motivated learning behavior. These findings underscore the crucial roles of intrinsic motivation and readiness for online learning in the success of online English learning. The implications of these findings are discussed in relation to developing English courses for adult online learners and reducing dropout rates in online education.