Density functional theory (DFT) study of adsorption of 0.25 monolayer of either Si or Con 4H-SiC{0001} surfaces is presented. The adsorption in high-symmetry sites on both Si- and C-terminated surfaces was examined and the influence of the preadsorbed 0.25 ML of hydrogen on the Si/C adsorption was considered. It was found out that for Si on C-terminated surface and Con Si-terminated the most favourable is threefolded adsorption site on both clean and H-precovered surface. This is contrary to the bulk crystal stacking order which would require adsorption on top of the topmost surface atom. In those cases, the presence of hydrogen weakens the bonding of the adsorbate. Carbon on the C-terminated surface, only binds on-top of the surface atom. The C C bond-length is almost the same for the clean surface and for one with H and equals to -1.33 A which is shorter by -0.2 than in diamond. The analysis of the electronic structure changes under adsorption is also presented. (C) 2015 Elsevier B.V. All rights reserved.