A molecular modeling program, SURFISS, is described which allows for the generation of representative configurations of surfactant molecules organized as planar monolayers at an air--liquid interface. The monolayer structures are generated by means of a conformational search optimization procedure using constraints derived from the data obtained in neutron reflectivity experiments. The utility of the program is demonstrated through successful simulations carried out for monolayers formed from the nonionic surfactant monododecyl hexaethylene glycol (C12E6) and the cationic surfactant hexadecyl trimethylammonium bromide (C(16)TAB).