The paper investigates maintenance practices of Enterprise Resource Planning (ERP) systems with emphasis on interactions between on-site maintenance team and third parties. A case study approach is used to identify maintenance activities in ERP systems, to classify them into maintenance categories, and to characterize dynamics and frequency of interactions with the third-party. Multiple deployments of the same ERP system are used in the investigation. Each deployment is in a different stage of its life-cycle what allows analyzing maintenance activities with regards to system’s maturity. Five common maintenance activities (corrective, adaptive, perfective, preventive, user support) are considered while main attention is devoted to a recently introduced category pertaining to ‘communication, coordination and knowledge exchange with external parties’ and named ‘external parties’. Interviews with maintenance staff are used to identify maintenance tasks and maintenance requests database is used for numerical analysis of maintenance requests. The results show that majority of maintenance tasks are performed by the on-site team and maintenance requests escalated to the third party require longer implementation lead times.