Infrared spectroscopy has been used to investigate the surface properties of anatase gels containing up to ca. 1% K2O, one of the additives most commonly employed to give TiO2 pigmentary characteristics. It was found that K2O mostly collects at the surface of the material, as revealed by the spectral behaviour of surface hydroxyls and of surface sulphate contaminants. During the thermal treatment leading to the pigmentary material, K2O progressively modifies TiO2 surface acidity, so that on the low-surface-area final product no Ti ions are revealed by suitable admolecules (pyridine, CO), but only coordinatively unsaturated K ions, acting as weak Lewis centres. The surface modifications caused by K2O are slowly reversible on contact with water vapour.