By putting into conversation theories of capital devaluation, the scholarship on the socioecological fix, and critical energy studies, this paper offers a conceptual framework to study the phase-out of fossil fuel energy infrastructure. The concept of moral devaluation is presented, which emphasises the interaction of economic, political, and socioecological dimensions shaping the main forces, constraints, and regulatory mechanisms to devalue emission-intensive fixed capital. This notion of moral devaluation is useful to inform empirical research on the institutional arrangements through which the devaluation of fixed fossil fuel capital is resisted and organised. Moreover, this notion complements current studies about the possibility of a capital switch to renewable energies as a socioecological fix to the climate and other entangled crises of capitalism, by emphasising that the devaluation of fixed capital represents a constraining condition for the rapid emergence of a socioecological fix based on fixed capital formation in the low-carbon energy sector.