This paper describes a method to study the reactivity of neutral dusters at single-collision-like conditions, which enables the determination of absolute numbers for the reaction probability (S) in a collision. A beam, of clusters is produced in a laser vaporization source and skimmed and passes a cell with reactive gas, in which the clusters experience one or a few collisions with the gas molecules, The reaction products are detected with laser ionization and mass spectrometry. The depletion of pure dusters and the appearance of products are evaluated with a statistical model providing S for the first, second, etc., molecule adsorbed. The O-2 and D-2 reactivity of Fe-n, Co-n, and Cu-n has been investigated for clusters in the approximate size range 10-60 atoms. The oxidation of transition metal clusters, here exemplified by Co-n and Fe-n, shows a simple S vs n dependence, where S increases almost monotonically as n increases from 10 to 20, while for larger n, S remains high and almost constant; S approximate to 0.7 for both Fe-n and Co-n. The low O-2 reactivity measured for the small transition metal clusters may be an effect of the products having a short lifetime due to the high exothermicity of the oxidation reaction. For copper clusters there are repeated minima in the O-2 reactivity appearing at duster sizes that are known to have high IP and closed electronic shells, Co-n is much less reactive toward D-2 than O-2, and S for D-2 on COn exhibits large size to size fluctuations. Cu-n and small Fe-n appear unreactive (detection limit S approximate to 0.02) toward D-2, whereas the larger Fe-n (n greater than or equal to 23) react with a low probability.