Project-based firms (PBFs) increasingly provide comprehensive solutions that consist of products, product systems and services. In solution businesses, long-term collaborative relationships between solution providers and customers are essential. However, little is still known about how relationship marketing activities should be integrated across organizational units, particularly at the practical level of delivering individual projects and services belonging to complete solutions. In this study, based on a case study of a project-based firm and four of its system delivery projects, we identify eight micro-level integration mechanisms for integrating the activities of the project and service business units at the level of delivering a single solution. The joint participation of both project and service business units in project and service activities over the life cycle of a single delivered system enhances the management of customer relationships between the units, and ensures the continuity of the customer relationship over the system life cycle. The identified integration mechanisms also help PBFs to integrate services into their core business and overcome the problems arising from the discontinuous nature of project business.