Sustainability is high on many research and policy agendas, and a number of specific models to measure sustainability have been developed. However, most models are based only on the first law of thermodynamics and, thereby, are incomplete and approximate. Sustainable processes are those whose rates are maintained over time without exceeding the innate ability of their surroundings to support the process. We present a model for measuring the sustainability of processes that adapts and integrates the first and second laws of thermodynamics and the concept of rate processes, thereby forming a new synthesis. The degree of sustainability of a process, whether ecological, economic, social, chemical, or biological, is expressed quantitatively in terms of algebraic equations. It is a dynamic approach that applies at any scale and takes into consideration the spatial and temporal factors of processes, thus permitting empirical applications that correspond to real world conditions (dynamic, complex, and evolving). These characteristics make it especially suitable for applications in the fields of chemistry, chemical engineering, and ecology.