The structure of clustered supersonic underexpanded jets of molecular nitrogen and argon was measured by the method of molecular beam mass-spectrometry. Peculiarities of application of the molecular beam methods for recording the supersonic rarefied gas jets under the conditions of weak and developed condensation (i.e., in the presence of small and large clusters in jets) have been discovered, identified, and studied. An unusual shape of longitudinal and transverse cross sections of the clustered supersonic jets was revealed and explained when scanning with a molecular-beam system. It has been determined that small clusters and monomers are the sources of double-ionized monomers available near the flow axis, and dimer ions at the early stages of condensation, whereas another mechanism of such ion formation dominates, when large clusters area available in the flow. A marker for fixing the stage of formation of small clusters in a supersonic flow is proposed.