The reaction of free radicals with organic molecules is an important pathway in many biological and environmental processes. Phosphorus and thiophosphorus esters are common functionality found in a wide range of compounds from pesticides to chemical warfare agents to biologically relevant molecules. The research that will be presented focuses on the kinetics and mechanism of the reaction of a series of phosphorus and thiophosphorus esters with oxidizing and reducing radicals in aqueous systems. The effect of the group Y, and pH on reactivity and mechanism will be discussed, as well as product distributions from the free radical reactions.