HAZOP (Hazard and Operability) studies began about 40 years ago, when the Process Industry and complexity of its operations start to massively grow in different parts of the world. HAZOP has been successfully applied in Process Systems hazard identification by operators, design engineers and consulting firms. Nevertheless, after a few decades since its first applications, HAZOP studies are not truly standard in worldwide industrial practice. It is common to find differences in its execution and results format. The aim of this paper is to show that in the Mexican case at National level in the oil and gas industry, there exist an explicit acceptance risk criteria, thus impacting the risk scenarios prioritizing process. Although HAZOP studies in the Mexican oil & gas industry, based on PEMEX corporate standard has precise acceptance criteria, it is not a significant difference in HAZOP applied elsewhere, but has the advantage of being fully transparent in terms of what a local industry is willing to accept as the level of risk acceptance criteria, also helps to gain an understanding of the degree of HAZOP applications in the Mexican oil & gas sector. Contrary to this in HAZOP ISO standard, risk acceptance criteria is not specified and it only mentions that HAZOP can consider scenarios ranking. The paper concludes indicating major implications of risk ranking in HAZOP, whether before or after safeguards identification. (C) 2013 Elsevier Ltd. All rights reserved.