A modular seat system for a passenger compartment of a vehicle, in particular an aircraft, with at least one seat which includes a seat unit with a seating element and a seat back, a seat frame with at least one pair of essentially vertical support elements, at whose upper end a bearing structure is arranged on which the seat unit rests, and a fixing structure in the floor of the passenger compartment with at least two longitudinal rails, which have a predetermined spacing from one another, for accommodating the lower ends of the support elements. In order to ensure a cost-effective and flexible fixing of standardized installation modules--in particular passenger seats--with different spacing of the mounting rails, there is provided a spacing device, which defines the spacing between the two support elements at their upper and/or lower end, so that the spacing of the support elements from one another is variable independently of the width of the seat unit.