The adsorption of isocyanides (R-N C) on the Si and Ge (100)-2 x 1 surfaces was studied by dispersion corrected density functional theory calculations. The molecular and adsorption characteristics of the isocyanides systematically depend on the substituents, both of which are effectively parametrized by the energy of C-lone pair molecular orbital. The stabilities of different adsorption geometries depend on the orbital energies in opposite directions. Consequently, most isocyanides with electron-donating substituents prefer C-dative configuration on both Si and Ge surfaces, while electron-withdrawing trifluoromethyl isocyanide on Si prefers a [1 + 2] cycloaddition structure. Analysis on the change in molecular orbitals upon adsorption offer chemical insights into bonding configurations. Our results confirm recent observations on isocyanide's adsorption on the Ge(100) surface, but suggest that Si(100) literature requires revision. Varying sign and magnitude of charge transfer to the surface were obtained by adsorption of isocyanides, and such ability to modulate the surface properties may be useful toward a wide application of functionalization by isocyanides. (C) 2016 Elsevier B.V. All rights reserved.