This work intends to assess the evaluation of analytical categories related to the identification of transaction costs. Assuming an analytical model for the identification of the transaction costs proposed by Arbage (2004) in which categories are discussed in terms of which analyses to be made and how to evaluate them. Amongst these categories, ‘opportunism’ is assessed in terms of patterns of behavior and measured in terms of confidence. The work supports the use of the fuzzy theory for measuring this category in particular. Through the fuzzy theory, there is the possibility of modeling and manipulate vague and inexact information mathematically, natural of the human language and, therefore, also the information supplied for the specialists when characterizing the considered processes.