This paper attempts to identify the reasons behind the adoption of the accounting information tools which has been implemented in Portuguese third sector organizations. The research was designed to face new institutional theory with rational choice theory. Data was collected from third sector organizations operating in Portugal. The questionnaire applied was based on earlier literature. Results support the theoretical perspective of institutionalism. Indeed, third sector organizations operating in Portugal use essentially traditional accounting tools like financial accounting statements, cost accounting and budgeting, and there are clear evidences of coercive isomorphic behaviours. Future research should consider a qualitative approach to deeply understand how and why these organizations use accounting tools.