The problem of simultaneous diffusion and reaction in an environment of a high-temperature polymer melt has been addressed under circumstances, in which micromixing plays a major role. Controlled Polypropylene degradation by peroxides has been taken as an experimental test case. In a previous study, degradation in a twin-screw extruder was observed to take place in a homogeneously mixed environment (Iedema et al., Chem. Eng. Sci. 56 (2001) 3659). In the present paper, we study a static mixer reactor, where the process is rate-controlled by micromixing. This follows from the clear deviation from the homogeneous situation and the fact that MWDs were strongly dependent on throughput. In order to model the mixing process, a laminar micromixing theory has been applied (Ottino, Chem. Eng. Sci. 35 (1980) 1377) in view of the high viscosity, employing the concept of 'striation thinning'. This is described by a parameter alpha, which is the basic parameter of the model, where better micromixing accords higher alpha. The experiments carried out at different polymer throughputs, leading to different MWD, can adequately be described by the micromixing model utilising alpha as the single parameter. They correspond surprisingly well to values found from a CFD-based study (Fourcade et al., Laminar striation thinning in static mixer reactors from computational fluid dynamics and laser induced fluorescence, Chem. Eng. Sci. (2001) in press), describing one-phase, Newtonian flow in a similar geometry. However, the throughput dependence of alpha shows a significant deviation from that of the Newtonian one-phase system. Still, the micromixing model proves to be practically valuable in describing micromixing controlled phenomena.