Fragmentation of small heteronuclear NapKn-p+ clusters (n = 3-7, p = 0-2) induced by collision with helium atoms is investigated at a laboratory energy of 4800 eV. Populations of the various fragmentation pathways are measured. Detailed analysis of the collision is performed with a multicoincidence technique allowing the determination of the velocity vectors of the fragments. Fragmentation is predominantly induced by momentum transferred in binary collisions between He and alkali cores, electronic excitation accounting only for about 10% of the process. Experiments with heteronuclear clusters allow the study of the various fragmentation mechanisms following the initial momentum transfer.Two-step fragmentation processes are governed by the endothermicity of the pathway while direct ejection of an alkali atom in binary collisions is directly dependent on the relative number of Na and K cores.