The present invention relates to an ionizer mounting structure for a vehicle air conditioning system. The air conditioning system includes an air conditioning case having an internal passageway and an ionizer for generating negative ions and positive ions within the internal passageway. The air conditioning case has a discharge electrode insertion hole and the ionizer has a plurality of discharge electrodes extending into the internal passageway through the discharge electrode insertion hole. The ionizer mounting structure includes a mounting and demounting structure for allowing the ionizer to be mounted to the air conditioning case through sliding movement from a demounting position on a passenger room side to a mounting position above the air conditioning case and for allowing the ionizer to be demounted from the air conditioning case through sliding movement from the mounting position to the demounting position.