<p>Research on online informal learning of English (OILE) has highlighted numerous benefits for learners’ learning experiences and outcomes. However, to fully utilise these benefits, it is imperative to scrutinise the mediating factors that may influence the actual realisation of OILE’s potential benefits, especially for marginalised English-as-a-foreign-language (EFL) learners. Drawing on the model of investment, this research explored the role of OILE in the EFL investment of two Chinese university students who have moved from rural to urban China for higher education. Through an analysis of qualitative data collected via interviews, diaries, and digital artefacts, this study found that OILE’s transformative potential is mediated by a dynamic interplay of socio-cultural factors. By unpacking this interplay, we illuminate the complex, non-linear relationship between OILE and EFL investment, challenging the predominantly optimistic narrative in the existing literature. This study concludes by delineating empirical, theoretical, and pedagogical implications for enhancing the language investment of disadvantaged groups like rural Chinese university students.</p>