Nowadays, we can observe a growing number of investment projects in the construction industry and environmental engineering, as well as a growing interest in socially responsible projects aiming to contribute to sustainable development. Interorganizational cooperation in engineering projects is associated with many opportunities and risks that continue to be a challenge for today’s world. The aim of this work is to identify and assess relationship-specific risks and risk determinants threatening to achieve the intended benefit of interorganizational cooperation and to develop assumptions for the risk management model in such projects. It will support sustainable project management in interorganizational engineering projects and increase their sustainability. In this work, 18 risks and 28 risk determinants were identified. Risks’ frequency of occurrence, consequences, possibility of detection and Risk Priority Numbers as well as the significance of risk determinants were assessed based on the data gathered from 12,352 interorganizational projects from 100 Polish companies. It was found that self-interest orientation, hiding internal problems and/or problems related to the joint venture, unfavourable and imprecise contract, and lack of a good project coordinator pose the highest risk in interorganizational engineering projects. The study revealed that the highest Risk Priority Numbers in interorganizational projects were generally obtained for construction projects, lower for socially responsible projects taken by engineering companies, and the lowest for environmental engineering projects.