Public policies have required public agencies to adopt sustainable public procurement. It is necessary to understand the importance of the requirement for sustainable purchases to happen. Thus, this study aims to examine the main difficulties in adopting SPP at a public university in Brazil. To that end, research on the topic was carried out based on critical theory using qualitative methodology through a case study. Semi-structured interviews were conducted with three groups selected intentionally: requesters, purchasers and suppliers. Aspects related to the understanding of sustainability, SPP and the relationship between ethics and sustainability were analysed. The study revealed that the interviewees understand that ethics is directly related to the practice of sustainability, but it is a difficult issue because of its historical roots and changes in paradigms. Due to the complexity of the SPP theme, one of the main implementation difficulties mentioned by the interviewees was the lack of training. For suppliers, the understanding of sustainability was already consolidated, but it required agreement between being sustainable and economically viable for the company. Thus, this work sought to expose how those involved in SPP understood, refuted or validated sustainability issues in government procurement. Previous studies revealed the difficulties and conceptions of buyers, but we did not find research that investigated requesters and suppliers. It is hoped that this information will serve actions in favour of sustainable production and consumption and as a basis for other studies on sustainable public procurement.