This study investigates how Greenpeace, a non-profit environmental organization, and Preem, a for-profit fuel company, construct sustainability in their external communication. Through Foucauldian discourse analysis, we explore the interests, values, and discursive strategies employed by Greenpeace and Preem and how they shape sustainability discourses. Our findings reveal distinct perspectives on sustainability: Greenpeace emphasizes vast and complex environmental issues such as climate change, pollution, and destruction of nature, advocating for radical societal changes and global solutions. Aligning with the dominant sustainability discourse, Greenpeace strategically leverages its legitimacy to promote an alternative discourse. On the other hand, Preem opposes the holistic discourse of Greenpeace, advocating for incremental reform and technological innovation within the existing politico-economic system. Thus, both organizations assert their authority and employ discursive strategies to legitimize their interests. Greenpeace portrays sustainability as a struggle, underscoring the need for collective action, while Preem presents sustainability as a journey, emphasizing individual responsibility in line with a neoliberal ideology. Our research contributes to our understanding of sustainability discourses in organizational communication. It sheds light on how different actors strategically construct and communicate sustainability messages, focusing on the ideological foundations, values, and interests that underlie these discourses. By enhancing our understanding of discourse transformation in shaping legitimate sustainability actions, our findings have implications for policymakers, researchers, and society at large.