The performance of a highly supersonic aerosol impactor as a size-discriminating instrument is explored experimentally with ultrafine particles having diameters as small as 51 Å. A hypersonic jet is formed by expansion of a gas-particle mixture through an orifice of diameter dn, from a source region maintained at a pressure po into an evacuated region kept at a background pressure p1 several hundred times smaller than po. Facing the jet perpendicularly, at a variable distance, L, from the nozzle exit, is a bounce-free flat target plate which collects a fraction, E, of the incoming particles. The collection efficiency, E(dp, L), is determined on-line by using monodisperse charged particles of diameter dp and measuring the electrical current they transport to the conducting target plate, which is grounded through an electrometer. The impactor shows sharp separation efficiency curves E(dp, L) only when L is smaller than some critical distance L*. For L > L*, the curves E(L) become non-monotonic as a result of some gas dynamic reasons not fully understood yet. Provisionally, over the limited range of pressure ratios explored, it appears that L* dn = 0.13 √( po p1). The size discrimination behaviour deteriorates also when L dn becomes smaller than one, though this lower limit seems to be somewhat more flexible and geometry dependent than the upper one. Within the region 0.8 < L dn < 0.13 √( po p1), E(dp, L) depends in a step-like manner on both these variables. The relatively sharp transition between E = 0 and E = 1 occurs roughly when the Stokes number in the impact region, S ∼ 32So L dn, is approximately one, where So is a standard Stokes number based on the nozzle exit diameter, dn, and the gas sound speed, co, at source conditions. For particles sufficiently small to be in the free-molecule limit, So = 0.1983 ρpdpco2 (dnpo), where ρp is their density. The fact that the fundamental variable S governing the impaction process is proportional to the product Ldp, makes these two factors conjugates of each other, and allows determining the diameter of an unknown monodisperse aerosol by measuring the value of L at which the capture efficiency E undergoes a step. This feature makes it possible to operate this impactor as an aerodynamic size spectrometer for ultrafine particles. With a pumping capacity of 500 l/min and an electrical noise level of 10-16 A, we have been able to 'measure' NaCl particles with a diameter as small as 51 Å, and to discriminate between particle diameters only 3-5 Å apart.