The reactions of neutral, ground-state Y (a(2)D, s(2)d(1)) with 1-butene, cis-2-butene, trans-2-butene, and isobutene were studied using crossed molecular beams. At a collision energy of 26.6 kcal/mol, four processes were observed for reactions with cis- and trans-2-butene and 1-butene, corresponding to nonreactive scattering of yttrium atoms and formation of YC4H6 + H-2, YH2 + C4H6, and YCH2 + C3H6. For the isobutene reaction, all channels but the YH2 product channel were observed. Experimentally measured product branching ratios indicate that YC4H6 formation is always the dominant process. However, for cis- and trans-2-butene and 1-butene, YH2, formation is significant. Formation of YCH2 was minor for all four isomers. At a collision energy of 11.0 kcal/mol, only YC4H6 + H-2 and nonreactive scattering were observed for all four butene isomers. A common mechanism is proposed for reactions of Y with all four isomers, involving formation of a pi-complex, followed by insertion of Y into a methyl C-H bond producing a methyl-substituted H-Y-allyl intermediate, followed by competition among three hydrogen migration processes each leading to a distinct product channel.