The reactions of ground state Y (a(2)D) with H2CO and CH3CHO were studied at a range of collision energies in crossed molecular beams. For reaction with H2CO, three product channels were observed: formation of YH2 + CO, YCO + H-2, and YHCO + H. Reaction with CH3CHO led to three analogous product channels involving formation of HYCH3 + CO, YCH2CO + H-2, and YCH3CO + H. The calculated CCSD(T) energetics and DFT geometries for key intermediates in both reactions, together with RRKM theory, are used to calculate a priori the branching ratios between various product channels. These calculated values are compared to those obtained experimentally.