Amplification factors of a particle multiplier having Cs-Sb dynodes made from an EMI 6260 photomultiplier by removing its photocathode were investigated for singly charged alkali ions of Cs, Rb, K, Na and Li in the energy range 0.5-5 KeV by considering the ratio of currents at the anode of the multiplier and an efficient Faraday cup placed before the multiplier. From curves of ion velocity against amplification factor, threshold velocities for secondary electron emission from oxidised Cs-Sb surface by these ions were measured at 0.45, 1.0, 1.2, 0.6 and 1.3*10⁴ m s^-1 respectively for Cs⁺, Rb⁺, K⁺, Na⁺ and Li⁺. There exists a linear dependence of gain factors on the velocity of the ions and the slopes of the secondary electron emission curves are proportional to the square root of the ion masses.