‘Transaction costs’ are commonly referred to in the recent literature on aid effectiveness. Aid transaction costs, however, have been neither consistently defined nor measured. This article defines aid transaction costs as all the economic costs associated with aid management that add no value to aid delivery. This enables the ‘net’ transaction costs that should be minimised to be identified. An analytical framework is then developed for assessing these costs. This allows the effectiveness of different aid modalities to be compared, according to the characteristics of the aid transaction. The article shows that the choice of aid modality should depend on these characteristics and, therefore, that the minimisation of transaction costs should not be an end in itself.