A range of continuous mixing machines were used as continuous reactors for grafting maleic anhydride onto polypropylene. The machines used were (1) a nonintermeshing modular counterrotating twin-screw extruder, (2) an intermeshing modular corotating twin-screw extruder, (3) intermeshing modular counterrotating twin-screw extruder, and (4) a Kobelco Nex-T continuous mixer. The grafting reaction of maleic anhydride onto polypropylene and degradation of polypropylene during the grafting reaction were investigated as means for comparing these different machines for reactive extrusion. The influence of processing variables such as screw speed and processing temperature on polymer characteristics also was investigated. Generally, in a comparison of the different machines, the intermeshing counterrotating twin-screw extruder had the lowest levels of grafted maleic anhydride, whereas the Kobelco Nex-T continuous mixer under the conditions used had the highest levels of grafted maleic anhydride. (C) 2003 Wiley Periodicals, hic. J Appl Polym Sci 90: 175-5-1764, 2003.