A strong reduction of the critical thickness of the spin reorientation transition in Ni/Cu(0 0 1) has been observed when covered with CO or H-2. For uncovered Ni films a critical thickness of 10.5 ML has been found at T = 300 K. The critical thickness is reduced by about 3 and 4 ML after adsorption of 0.5 ML CO and H-2, respectively. The observed shift in the spin reorientation transition implies a reduction of the negative surface anisotropy energy by about 50% in both cases.