Reactive polymers each bearing large numbers of reactive groups are important for new compatibilized blends with the graft copolymers generated in situ. Potential of gelation and phase homogeneity are two important issues. A case study on cellulose acetate and poly(styrene-co-maleic anhydride) (SMA) was reported here to demonstrate the two issues. It was found that phase homogeneity appears when the grafting conversion of SMA exceeds around 50% for the dispersed SMA phase. There is a relatively narrow range of SMA grafting conversion for the formation of compatibilized ternary mixtures with a homogeneous phase size and a reasonable weight-average molecular weight. The study shows the importance of polydispersity in promoting phase homogeneity due to the weighting towards more grafting of higher-molecular-weight fraction of the reactive polymers.