The rapid growth of the Internet in the recent past has encouraged global deployment of work by an increasing number of organizations around the world, and they are now in a better position to outsource their IT functions to specialist vendors. With the passage of time, we find more and more software systems moving into the maintenance phase. Such software systems have become an increasingly significant expenditure for businesses. Consequently, these are often potential candidates for outsourcing. Inadequate information regarding the size, complexity, reliability, maintainability, etc., of these systems often makes the task of estimating the maintenance effort a challenge. Other human and organizational factors, typical to maintenance activities, such as organization climate, customer attitude, engineers' attitude, the need for multi‐location support teams, etc., make the situation even more complex. In this paper we present the results of an empirical study carried out to identify such factors and study their influence on the maintenance effort. We classify these factors in four categories, namely system baseline, maintenance team, customer's attitude and organizational climate. We also propose a model which can help a practitioner to predict and control the impact on maintenance effort, based on the strengths of these factors. Copyright © 2006 John Wiley & Sons, Ltd.