We empirically study personal norms of sustainability, conceptualized according to the norm-activation theory and operationalized under the notion of strong ecological-economic sustainability, for commercial cattle farmers in semi-arid rangelands of Namibia, a system that is subject to extensive degradation. We characterize farmers’ personal norms, study their determinants, and analyze their impact on actual management based on the dual-preferences model. We find personal norms of sustainability that are heterogeneous across farmers but vary little with socio-demographic or environmental characteristics. We find no evidence for a significant impact of personal norms on actual management behavior, which may be due to farmers not feeling capable for averting adverse long-term consequences of their management. This may contribute to the observed degradation of rangelands in Namibia.