Software sustainability is an increasing issue in software engineering and we aimed to see how it is understood in research software engineering. By interviewing research software engineers (RSEs) we aimed to investigate how the research software engineering community understands and measures software sustainability and if they adopt sustainable practices in their own work. We discovered that current practices fall well short of the defined software engineering principles and practices and there’s a fundamental gap between basic software engineering practice and RSE activity. This gap needs to be closed in a drive towards achieving sustainable software. We suggest the creation of a set of practices to be used by RSEs to guide them through the process of testing the sustainability of software and make them more recognisable. For future work, we suggest exploring the possibility of raising awareness of the Karlskrona Manifesto for Sustainability Design and provide some education and training in it for RSEs.