This study aims to identify critical success factors for complex projects developed in alliances between companies. The theoretical framework is grounded in concepts related to projects and partnerships that address aspects related to motivation, characterization and dynamic operation. The exploratory research was conducted in an engineering services organization, mixing methods of qualitative and quantitative research. Data were gathered from 11 projects in partnerships via interviews performed with 43 managers involved in such projects. The combination of the literature review and the field research evidence highlighted some critical success factors in the projects of Engineering, Procurement and Construction (EPC) that were developed in the alliances. The results indicate five factors that were strongly correlated: cooperation, trust, information exchange, synergy and degree of integration. Furthermore, from 17 factors, 12 were significantly related to the projects' results in some of the dimensions studied.