Multiple methods for initiation and selecting catalyst concentration exist in atom transfer radical polymerization (ATRP). Among them, simultaneous reverse and normal initiation (SR&NI) ATRP and initiators for continuous activator regeneration (ICAR) ATRP are phenomenologically very similar. In both methods, thermal radical initiators are employed to reduce the catalyst in the higher oxidation state and generate Cu-I activator in Situ. SR&NI and ICAR ATRP generally differ in the amount of catalyst used and in the rate of catalyst reduction. Commonly, SR&NI ATRP requires high catalyst loadings and a quick initial reduction of Cu-II, while ICAR ATRP relies on slow and continuous reduction of smaller catalyst loadings. However, these criteria might not be sufficient to universally distinguish among both techniques. This article investigates both methods through kinetic simulations and establishes a borderline kinetic criterion. If the polymerization rate depends on the rate of decomposition of the radical initiator, the system follows ICAR ATRP kinetics, and if it depends on the ATRP equilibrium constant, it follows SR&NI ATRP. The transition from one to the other mechanism occurred continuously with an inflection point at a ratio of rate coefficients of radical initiator decomposition to propagation of about k(dc)/k(p) approximate to 10(-7) M under typical conditions. For faster initiator decomposition and slower propagation ATRP follows SR&NI ATRP, and for slower decomposition and faster propagation it obeys ICAR ATRP kinetics. The analysis to verify which mechanism is in operation is helpful for designing reaction conditions in order to obtain well-defined products.