The chemisorption and reaction of 3-aminopropyltrimethoxysilane (APTS) molecule on the Si(1 1 1)-(sqrt(3) × sqrt(3)) surface are investigated by first principles density-functional calculations within the generalized gradient approximation. Before studying the chemisorption of APTS molecule on the surface, we have firstly put three -OH groups on the silicon surface, but considering six different locations for H and O atoms. Upon their relaxations, model II, which is assumed to be crosswise for initial orientation of -OH groups, was found to be energetically more favorable than the others. In model II, after the relaxation, its conformation was transformed to skewed structure due to the repulsive forces between -OH groups. Isolated APTS molecule was also investigated in order to obtain its most stable molecular geometry, for which the HOMO-LUMO gap was found to be 4.41 eV. In addition to these, after the energetically most favorable hydroxylated Si(1 1 1) surface was calculated, APTS molecule was chemisorbed on the surface by means of liberating its methoxy groups. In our model for the binding of APTS molecule on the Si(1 1 1) surface, the silicon atom in the APTS forms three bonds to hydroxyl groups at the surface. © 2007 Elsevier B.V. All rights reserved.