An overview is given of the experimental evidence for and against the generation of radical intermediates in the reaction of sMMO with methane and related hydrocarbon substrates. The ability to derive unambiguous mechanistic interpretations from substrate probe experiments remains a challenging area of research. A consensus has evolved concerning experimental observations but not the deductions based on these observations. There is agreement that, if transients are formed in sMMO hydroxylations, they are quite short-lived and involve C-H bond homolysis. it is also apparent that some type of cationic species forms in many mechanistic probe studies.