Abstract: An adaptor system for mounting a gutter mount-type accessory on a rail mount-type vehicle's rooftop is disclosed. The system includes plural adaptor assemblies each including an extruded bracket member having a planar bottom surface for mating the upper surface of a mounting rail, and an edge region including first and opposing shoulder regions for secure gripping by a depending strut member and a hooked member that form a part of a gripping means in a rooftop accessory or accessory carrier. Preferably the first shoulder region has a first recess extending a predefined distance along and in parallel with the edge region, with the first recess being dimensioned to receive partly therein the depending strut. The second shoulder region preferably is defined as an upper surface of a second recess extending along and in parallel with the edge region for receiving partly therein a hooked member that forms another part of the gripping means.