The spin-reorientation transition of ultrathin Ni films is investigated by means of in situ magneto-optical Kerr imaging as a function of the temperature. A critical thickness d_c of about 11.4 monolayers (ML) at T = 170 K and 10.3 ML at 370 K has been found. Adsorbates strongly reduce this critical thickness. In particular the adsorption of about 2 langmuir H₂ reduces d_c to 7 ML at 170 K. The magnetocrystalline anisotropy energy of the clean Ni surface K_2s = -153 m eV/atom is strongly reduced by hydrogen adsorption.