The design and optimization of chemical processes in which reactions occur require a quantitative model for the reaction kinetics. Herein we describe two different approaches for developing such models for supercritical water oxidation (SCWO) processes. One approach is to develop empirical global rate laws that correlate a large set of experimental kinetics data. These rate laws have no connection with the governing reaction mechanism, and they must be extrapolated with caution. A second approach is to use the underlying mechanism as the basis for a quantitative kinetics model. Such a detailed chemical kinetics model (DCKM) requires a good understanding of the mechanism, but the validated model can then be used with confidence in engineering applications and to learn more about the detailed chemistry. We illustrate each of these approaches by exploring the oxidation of phenol in supercritical water.