The thermal stability of initiators used in free radical polymerization has been well documented. Initiators are often used in solution, though, with relatively long feed processes to help control heat of polymerization. In some cases, the stability of initiator solutions is limited, especially at higher concentrations, and runaway reactions can potentially be severe. This paper reviews the chemical reactivity of concentrated initiator solutions and considers PHA studies and feed tank design to help ensure safe handling and manufacture.