Abstract: A supplementary fin for attachment to the blades of dispersion fans in microwave ovens is disclosed. Supplementary fins 7 and 7a are removably attached to the underside of respective blades 2 and 2a of standard dispersing fan 1. To maintain balance of the fan assembly, supplementary fins 7b and 7c are attached to the top side of respective dispersing fan blades 2b and 2c. The removable attachment capability of the supplementary fins is accomplished by sliding of tongue 5 through a rectangular hole 4 so as to align a fixing hole 6 with a circular hole 3. An expandable clip 9 is inserted through holes 3 and 6 and is expanded by insertion of a pin 8 so as to removably attach the supplementary fin 7 to dispersing fan blade 2. This removable attachment capability allows optimum dispersion characteristics to be achieved for a variety of dispersing fans, fan motors, and oven air venting configurations, by simple replacement of supplementary fins with supplementary fins of different design.