The two reactive systems using cellulosic fiber reinforcement emphasized in this article are twin screw extrusion maleation of polypropylene (PP) and structural reaction injection molding (SRIM). Both simultaneous and sequential compounding of sweet gum wood fiber (WF) with maleation were studied. Similar PP/WF mechanical properties are observed for simultaneous and sequential compounding. However, the melt viscosity of the simultaneous compounding and maleation deceases initially compared to neat polypropylene, whereas the sequential process does not have an initial decrease. After the initial response, the melt viscosity and shear thinning characteristics in both systems increase with WF content compared to their respective low WF behavior. The cellulosic reinforcement in the SRIM studies include: cheesecloth; and nonwoven mats formed separately from fiber bundles of sugar cane rind, kenaf, and sweet gum wood. In the SRIM system the modulus and tensile strength of the neat polyurea/urethane increases with cellulose content, independent of cellulosic source; the modulus doubles with only 4% cellulose, and the tensile strength doubles with only 7% cellulose. However, the elongation to break decreases from 300 to 30% with only 4% cellulose. Cellulose is as reactive with isocyanate in SRIM as are the commercial polyols.