  A new experimental technique to study the decays of $B^0_s$ mesons into final
states involving unreconstructable particles is proposed. The method uses a
characteristic feature of fast flavour oscillations of $B^0_s$ mesons. Analysis
of the oscillation-frequency spectrum of partially reconstructed flavour-tagged
$B^0_s$ decays provides information about the mass spectrum of unreconstructed
decay products and suppresses any background that does not come from the decays
of $B^0_s$ mesons.
