This paper concerns the integration of Safety Management Systems of the Estonian Seveso II establishments into ISO-based Integrated Management Systems (IMS). Estonia has an obligation to follow the requirements of the EU Seveso II Directive since joining the EU in 2004. Today altogether 51 enterprises with major accident hazards are in the official register. According to the data available at the Estonian Association for Quality, seventeen of these establishments have certified quality (ISO 9001), environmental (ISO 14001) management integrated systems. Therefore it is useful and politically sound to integrate the SMS to the IMS. The essential goal of the current study is to explain the application of the unified management system and to find characteristic features for further recommendations. The results have demonstrated that, in spite of different approaches, a number of common factors exist, enabling general recommendations to be worked out.