The details of an unscheduled polymerization of acrylic acid initiated by indirect heating have been characterized by in-depth analysis of the polymer, kinetic and thermodynamic calculations, and heat and material balances on the system. The data fit together to give a consistent picture of the polymerization scenario. The fact that the acrylic acid polymerization was not violent was primarily due to the presence of about 10 ppm phenothiazine, which delayed the onset of free radical polymerization until about 2 to 3 hours after the monomer had reached its boiling point. Since much of the acrylic acid was converted to dimer during this induction period, the ultimate energy release was greatly limited. Other moderating factors were a relatively slow heatup of the tank contents in the external heat exchange recycle loop, free vapor venting, and the incursion of fire monitor water into the tank.