 MIXSAR is an open-source R package designed to facilitate the development and application of tracer mixing models. It provides a comprehensive framework for implementing Bayesian tracer mixing models, including the ability to specify informative priors on source-proportion contributions, combine sources, and incorporate fixed and random effects as covariates. This versatile tool can be used to analyze a wide range of environmental studies from simple two-source models to complex multi-source models. This article was authored by Brian C. Stock, Andrew L. Jackson, Eric J. Ward, and others.