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Oxidative water purification of micropollutants (MPs) can proceed via toxic intermediates calling for procedures for connecting degrading chemical mixtures to evolving toxicity. Herein, we introduce a method for projecting evolving toxicity onto composite changing pollutant and intermediate concentrations illustrated through the TAML/H 2 O 2 mineralization of the common drug and MP, propranolol. The approach consists of identifying the key intermediates along the decomposition pathway (UPLC/GCMS/NMR/UV-Vis), determining for each by simulation and experiment the rate constants for both catalytic and noncatalytic oxidations and converting the resulting predicted concentration versus time profiles to evolving composite toxicity exemplified using zebrafish lethality data. For propranolol, toxicity grows substantially from the outset, even after propranolol is undetectable, echoing that intermediate chemical and toxicity behaviors are key elements of the environmental safety of MP degradation processes. As TAML/H 2 O 2 mimics mechanistically the main steps of peroxidase catalytic cycles, the findings may be relevant to propranolol degradation in environmental waters., Competing Interests: Y.S. and T.J.C. are coinventors on a patent for the latest generation TAML catalysts, allowed in the United States and pending elsewhere—the connection to this work is remote. T.J.C. is the Creator Founder and a Board member of Sudoc, LLC, formed to commercialize applications of the latest generation TAML catalysts. T.J.C.'s Irrevocable Family Trust is an owner of significant holdings in Sudoc, LLC—the vast majority of the Trust's future income is directed to supporting research and education in sustainability science, especially to advancing the rational handling of endocrine disrupting chemicals in our global civilization., (© 2020 The Author(s).)