Reactions of ethylene molecules on Cr-n(+) (n = 1-4), CrnO+ (n = 1-4), and CrnOH+ (n = 1, 2) were studied in a tandem mass spectrometer equipped with a reaction cell. The mass spectra of the product ions showed that (1) under a single collision condition, oxidation of ethylene and chromium pickup (Cr-pickup) by ethylene take place on Cr-n(+) (n = 2-4), CrnO+ (n = 2-4), and CrnOH+ (n = 2), and (2) under a multiple collision condition, two ethylene molecules are chemisorbed exclusively on CrO+ and CrOH+; the ethylene molecules seem to be dimerized in comparison with our preliminary density functional theory calculation. Size and collision-energy dependences of the reaction cross sections lead us to conclude that the Cr-pickup reaction proceeds via an intermediate in which C2H4 is chemisorbed on a parent ion. The preferential ethylene dimerization on CrO+ and CrOH+ implies that, on a silica-supported chromium catalyst (Philips catalyst), ethylene polymerization proceeds preferentially on active centers consisting of atomic chromium bonded to O or OH of the substrate.