baking powder (measuring equivalent): for each tsp of baking powder needed use: 1/2 tsp cream of tartar, 1/4 tsp baking soda, 1/4 tsp cornstarch or arrowroot. Mix only as much as you need per recipe since it will lose potency during storage. (source: Rodale's Basic Natural Foods Cookbook)(Cream of tartar is the common name for potassium hydrogen tartrate, an acid salt that has a number of uses in cooking)