An increasing number of institutional practices has emerged to rule or improve organizational performances and reduce transaction-costs. However, these practices don´t necessarily enhance efficiency. The neo-institutional theory is chosen as the groundwork for this study in order to avoid a reductionist, mechanical analysis, and also for taking into account the following aspects: legitimacy; efficiency; isomorphism; and deinstitutionalization. The ISO organization represents an interesting subject to study since it is legitimated worldwide, and transforms several values and beliefs into organizational rules and norms that become myths and ceremony. Moreover, this research aimed at analyzing two seemingly opposite elements in the institutional theory: efficiency and legitimacy. In order to accomplish its objectives, the study applied as reference the perception of specialists in relation to the efficiency of implementation of the international standards for quality management systems known as ISO 9001. This ISO seems to legitimate organizations that become certified and, at the same time, it´s believed to be a management model intended to produce more efficiency. As an outcome of the study, the dilemma efficiency versus legitimacy in the practice of ISO 9001 is analyzed, and then possible evidences of a process of deinstitutionalization are presented: innovative practices; more extensive standards; and modification of institutional values. Therefore this study represents a contribution to academic matters through applying the institutional theory to the practice of ISO 9001, in as much as to organizations, since it presents possible improvements in the implementation of this standard, and the tendency of its possible substitution.