Social media is widely used by accounting firms to achieve a variety of business objectives and is a key enabler of non-market strategies. Socio-political involvement (SPI) involves firms taking positions on issues that lack societal consensus, have low information rationality, evolving viewpoints and issue salience, with no clear performance outcomes for firms. SPI may result in firms alienating stakeholders with opposing views, resulting in no or adverse performance outcomes. Accounting firms traditionally may not engage with such issues due to the associated reputational risk. Further, research suggests firm size is one of the most prominent firm-level antecedents of socio-political engagement. As the empirical context for this paper, we choose the Brexit referendum, a significant historical but divisive event with contested social norms. This paper explores the engagement of accounting firms on Twitter with the #Brexit discourse from the referendum announcement to one month after the vote. The objectives of the study are to understand the nature of non-market socio-political engagement by accounting firms in the #Brexit discourse on Twitter, and explore the differences between accounting firms of different size in this context. Our findings suggest that accounting firms engaged in the #Brexit Twitter discourse through a variety of non-market socio-political engagement activities, and that smaller firms tended to engage more than larger firms, most likely reflecting the ideological inclination of firm management. The engagement of accounting firms in socio-political discourse extends our understanding of how accounting firms of all sizes use social media and supports critical accounting and institutional perspectives.