The maintenance of information systems is one of the greatest problems in the software life cycle: It is the most conflictive, costliest, less planeable; and the process requiring the most resources. In spite of this reality, most organizations do not possess methodologies for the maintenance. The imperative need of controlling the maintenance process, have carried us to propose a maintenance methodology. In this paper an adaptation of the ISO/IEC 12207 standard for the processes of the life cycle maintenance is presented. This methodology is being used by ATOS-ODS, one of the most important European consultants on software outsourcing and maintenance.