Disclosed herein is a method for improving the physical properties of cellulose fiber-containing material/PVC composites comprising adding to said composites an effective amount of at least one compatibilizer comprising an organometallic zirconium compound. The preferred organometallic zirconium compound is cyclo[dineopentyl(diallyl)]pyrophosphato dineopentyl(diallyl)zirconate. Articles of manufacture comprising such composites are also disclosed.