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Abstract: The reaction of yttrium (2D) with ketene (H2CCO) examined recently in crossed molecular beam experiments has been studied at B3LYP and CCSD (T) level. After the formations of different π-complexes with Y binding to Ch name="dbnd6" />O, Ce:glyph name="dbnd6" />O sites, respectively, several reaction channels are open. Two possible H-elimination pathways have been identified and they were found to correlate with singlet ground state and triplet excited state product, 1 YCHCO and 3 YCHCO, respectively. Three possible H2-elimination pathways have been identified, and the calculation results indicate that the two concerted pathways can be competing processes at low energy, while the step-wise mechanism needs higher collision energy. The experimentally observed products YCCO were found to exist in two different forms (cyclic and linear). The calculated barrier of Y insertion into Cading to the dominant product YCH2 at low collision energy in CMB study is only 2kcal/mol and the potential energy surface of this reaction is flat. In addition, the insertion of Y into Cs also identified, but this reaction is correlated to the product (vinylidene H2CC) with excited state, agreeing with the fact that the formation of YO+H2CC was not observed in experiment. [Copyright &y& Elsevier]