A theoretical study on the reaction mechanism of the silastannation of various alkynes with a palladium catalyst is presented. We investigate the different regioselectivities in the reactions of SiH3SnH3 with monosubstituted acetylenes (CN, H, CH3, and OCH3) catalyzed by Pd(PH3)(2). The overall reaction scheme is first examined, and then the factors for regioselectivities are analyzed. The rate-determining step is the insertion of acetylene into the Pd-Sn or Pd-Si bond of the complex. Three factors are pointed out as governing the reactivity and regioselectivity. The first is the electronic factor which determines the relative stabilities of the transition states (TS) involving differently oriented acetylenes and those of the regioisomeric intermediates obtained after the TS. The overall reactivity is determined by the electron back-donation from Pd (home) to acetylene (lumo), while the stable orientation of the substituted acetylene is determined by the electron donation from the home of acetylene to the lumo (localized on Sn or Si) of the Pd complex. The second is the steric hindrance of the ligands. The steric repulsion of PPh(3) of a Pd(PPh(3))(4) catalyst is large enough to give a different isomer from the one predicted by the electronic factor. The third is the occurrence of the thermodynamic control when the products after the TS are unstable and therefore the reverse reactions can easily occur. The regioselectivities reported experimentally and predicted theoretically here are reasonably explained by these three factors. The electron density distributions show electron donation and back-donation, supporting the electronic mechanism proposed in this paper. An agostic interaction, between palladium and hydrogen, is found in the intermediate after the insertion step.