The addition reactions of silylenoids H2SiLiX (X = F, Cl) to acetylene were studied by ab initio molecular orbital theory. The reaction mechanisms were revealed for the first time. The structures of reactants, transition states, and products were located and fully optimized at the G2(MP2) level. Based on the MP2(full)/6-31G(d) optimized geometries, harmonic vibrational frequencies of various molecules were obtained. The reaction paths were investigated and confirmed by intrinsic reaction coordinate (IRC) calculations. The barriers for the addition reactions were computed to be 77.3 (X = F) and 11.4kJ/mol (X = Cl) at the G2(MP2) level, respectively. Changes (DeltaS, DeltaH, and DeltaG) in thermodynamic functions, equilibrium constant K(T), and preexponential factor B(T) and second-order reaction rate constant k(T) in Eyring transition state theory were calculated over a temperature range of 100-1000 K, and then thermodynamic and kinetic properties of the reactions were analyzed.