Branching ratios between C-C and C-H bond activation were measured for reactions of ground-state Y (a(2)D, s(2)d(1)) atoms with two C3H6 isomers (cyclopropane and propene) in crossed molecular beams. For both isomers, C-C bond activation led to formation of YCH2 + C2H4 whereas C-H activation led to YC3H4 + H-2, and YH2 + C3H4. The angular and velocity distributions for all three product channels and for nonreactive collisions were measured at several collision energies (E-coll). For Y + cyclopropane, the branching ratio for YCH2 + C3H4 increased relative to YC3H4 + H-2 with increasing E-coll, this C-C activation channel becoming dominant at E-coll greater than or equal to 19 kcal/mol. For the propene reaction, phiYCH(2)/phiYC(3)H(4) also increased with E-coll, reaching 0.75:1.00 at E-coll = 28.8 kcal/mol. For both C3H6 reactants, formation of YH2 + C3H4 was observed as a minor channel at the highest collision energies. Experimental results and Rice-Ramsperger-Kassel-Marcus (RRKM) modeling indicate that for propene reactions all three channels involve initial formation of pi-association complexes that undergo insertion into one of the sp(3)-hybridized beta-C-H bonds in the methyl substituent. Decay of the yttrium allyl hydride intermediate by beta-H migration leads to the two C-H activation products, YC3H4 + H-2 and YH2 + C3H4. We propose that the YCH2 + C2H4 channel involves reverse beta-H migration forming the same strongly bound metallacyclobutane intermediate formed in the Y + cyclopropane reaction.