Abstract: An air vent cover system for mounting within a duct has a pair of side walls, a top wall, and a bottom wall. The air vent cover system comprises a housing with an opening. A tensioning mechanism releasably maintains the housing within the duct by applying an outward force on two opposite walls of the duct causing the other two opposite walls to move inward against the housing. A non-pivoting, stationary grate is secured to the housing with the stationary grate having horizontal and vertical bars and the bars spaced from each other to maintain a child-sized hand from going between the bars. A cover grate is pivotally mounted to the housing and has a plurality of semi-circular slits for allowing maximum amount of air to pass therethrough. A latching mechanism secured to the housing and cooperating with the cover grate releasably maintains the cover grate against the housing.