For the safe and trouble-free operation of a manufacturing plant and the safe storage of acrylic monomers a sufficiently effective polymerization inhibitor is necessary. As stabilizers different radical interceptors are used. Hydroquinone monomethyl ether (MeHQ) and phenothiazine (PTZ) are the standard stabilizers. In this paper, the decomposition kinetics of PTZ was investigated under process conditions. In acrylic acid a linear PTZ consumption was detectable under process conditions, whereby five different decomposition products were formed. The PTZ consumption was caused by thermal decomposition and by radical and oxidation reactions. With increasing temperature the portion of the total PTZ consumption caused by radical reactions decreased rapidly (68% at 60 degrees C -> 38% at 90 degrees C) and the part of the oxidation reactions of the inhibitor increased (27% at 60 degrees C -> 54% at 90 degrees C). Comparative investigations in air and nitrogen atmosphere resulted in different values for PTZ consumption and radical formation rates. Measurements of oxygen and PTZ consumption in air atmosphere showed a ratio of 2:1 mol mol(-1) (60 degrees C) which increased with temperature up to 4:1 mol mol(-1) (90 degrees C). The data showed that in acrylic acid stabilized with PTZ the oxygen consumption could not be totally prevented. This indicates that one part of the oxygen is consumed by the oxidation of PTZ while another part reacts directly with the primary radicals which are not trapped by the inhibitor. With the results of this work it is possible to optimize the PTZ stabilization of acrylic acid under process conditions in the presence and absence of oxygen.