The principle of hard and soft acids and bases is applied in the local. sense to rationalize the regiochemistry in the cycloaddition reactions of a few typical 1,3-dipoles, in particular those with phosphorus-containing dipolarophiles. Local softnesses are calculated using density functional theory. It is observed that the regioselectivity can be explained using these new reactivity descriptors based solely on the properties of the reactants. (C) 1998 John Wiley & Sons, Inc.