Abstract: Absolute cross-sections for dehydrogenation of an ethylene molecule onto 
, TinO+ (n = 2−25), 
, 
and 
were measured as a function of the cluster size, n, in a gas-beam geometry at a collision energy of 0.4 eV in an apparatus equipped with a tandem-type mass spectrometer. It is found that (i) the di-dehydrogenation proceeds on 
and TinO+, and the mono-dehydrogenation on 
, 
and 
, (ii) the di- dehydrogenation cross-section increases gradually with the cluster size of 
and TinO+, (iii)the mono-dehydrogenation cross-section increases rapidly above a cluster size of ∼ 18 for 
, ∼13 and ∼ 18 for 
, and ∼ 10 for 
, and (iv) therapid increase of the cross-section for 
(M = Fe, Co and Ni) occurs ata cluster size where the 3d-electrons start to contribute to the highest occupied levels of 
. These findings lead us to conclude that the 3d-electrons of 
lay a central role in the dehydrogenation on 
.
