The principle of hard and soft acids and bases has been applied in a local sense to rationalize the regiochemistry in cycloaddition reaction of a few typical 1,3-dipoles with phosphorus-containing dipolarophiles. It has been observed in most cases that the transition state with higher hardness is associated with lower activation energy. The hardness profile has also been investigated for these cycloaddition reactions; while the hardness value goes through a minimum along the reaction coordinate, its minimum does not coincide with the energy maximum.