Though the neural basis of working memory (WM) capacity is often studied by exploiting inter-individual differences, capacity may also differ across memory materials within a given individual. Here, we exploit the content-dependence of WM capacity as a novel approach to investigate the oscillatory correlates of WM capacity, focusing on posterior 9-12 Hz alpha activity during retention. We recorded scalp electroencephalography (EEG) while male and female human participants performed WM tasks with varying memory loads (2 vs. 4 items) and materials (English letters vs. regular shapes vs. abstract shapes). First, behavioural data confirmed that memory capacity was fundamentally content-dependent: capacity for abstract shapes plateaued at around two, while the participants could remember more letters and regular shapes. Critically, content-specific capacity was paralleled in the degree of attenuation of EEG-alpha activity that plateaued in a similar, content-specific, manner. While we observed greater alpha att...