This study explores the factors influencing AI adoption in professional service firms, specifically auditing firms of varying sizes in Australia. Grounded in the Technological-Organizational-Environmental (TOE) framework, the research employs a qualitative, multiple case study approach, examining three auditing firms: a Big Four international firm (BIG), a second-tier firm (MID), and a small local firm (SML). The findings reveal six key factors affecting AI adoption: technology affordances and constraints, innovation management, AI readiness, regulatory environment, and competitive environment. These factors vary significantly among the three firms. Larger firms, operating in an environment with high AI penetration, primarily perceive the operating affordance of AI, leading to greater scale and depth of adoption compared to smaller firms, which face weaker AI readiness and regulatory challenges. The study contributes to the literature by offering a holistic perspective on AI adoption in professional services, highlighting the need to reconsider a one-size-fits-all approach and the importance of firm size in shaping AI adoption strategies.This study explores the factors influencing AI adoption in professional service firms, specifically auditing firms of varying sizes in Australia. Grounded in the Technological-Organizational-Environmental (TOE) framework, the research employs a qualitative, multiple case study approach, examining three auditing firms: a Big Four international firm (BIG), a second-tier firm (MID), and a small local firm (SML). The findings reveal six key factors affecting AI adoption: technology affordances and constraints, innovation management, AI readiness, regulatory environment, and competitive environment. These factors vary significantly among the three firms. Larger firms, operating in an environment with high AI penetration, primarily perceive the operating affordance of AI, leading to greater scale and depth of adoption compared to smaller firms, which face weaker AI readiness and regulatory challenges. The study contributes to the literature by offering a holistic perspective on AI adoption in professional services, highlighting the need to reconsider a one-size-fits-all approach and the importance of firm size in shaping AI adoption strategies.