Information Systems (IS) literature shows a huge interest for open source technologies as well as from IT (Information Technology) strategies, business models or organization. Nevertheless, research on the application of open source in safety-critical information systems is particularly scant. By means of an exploratory study, this article describes the reasons why Thales, a firm specialized in safety-critical fields (defense, aircraft industry and security), has adopted open source software technologies. From a theoretical perspective, this articles relies on the technology, organization, environment (TOE) framework and the literature on technology and innovation adoption. This research suggests a model of barriers and motivations to open source software adoption for safety-critical information systems design.