Abstract The vapour deposition of Cs on MoS 2(0001) and the chenusorption of chlorocarbons on Cs/MoS 2 have been characterized by SIMS under low primary ion flux conditions. The Cs + intensity passes through a maximum at approximately 0.25 τ s, where τ s is the dosing time required for Cs saturation, and thereafter declines towards a lower, constant value. Cs deposition leads to an enhancement of all negative ion intensities to the extent that their combined yield may approach unity per incident primary Ar + ion and may contribute to the ion-induced crystal current (ICC) variation. The chenusorption of CH 2Cl 2 1,2-C 2H 4Cl 2 and trans-1,2-C 2H 2Cl 2 on the cesiated basal plane of MoS 2 is studied at 330 K. Elimination of chlorine occurs in every instance with loss of the organic molecular fragment into the gas phase. Chlorine uptake by Cs/MoS 2 leads to a progressive decline in negative secondary ion intensities. Chemisorption proceeds rapidly, particularly at high Cs coverages where it is essentially complete after reagent exposures of (1−2) × 10 −6 mbar.s.