MECHANISMS OF POSITRON ANNIHILATION ON MOLECULES G. F. Gribakin Two mechanisms of positron annihilation on molecules are considered: (i) Direct annihilation similar to that for positrons on atoms; for room temperature positrons it gives annihilation rates up to Z_eff about 1000. (ii) Resonant annihilation which proceeds via positron capture into vibrationally excited quasibound states of the positron-molecule complex. The latter is sensitive to the chemical composition of the molecule and can give very high annihilation rates (Z_eff up to 10^8). Both features agree with experimental data by the group of Surko.