The pollution of surface, ground and drinking water by pharmaceuticals in low concentration is anew emerging problem. Zinc oxide nanoparticles of different sizes have been characterized by XRD, TEM, UV-Vis/ DRS and N-2 adsorption-desorption measurements. The photocatalytic degradation of low concentrated pharmaceuticals tetracycline and ibuprofen at low photocatalyst amount and at low photocatalyst to substrate mass ratio (cat-to-sub ratio) have been investigated in details. The influence of pH value, ZnO and drug concentrations as well as the contribution of adsorption has been deeply studied. The formation of intermediates has been investigated by electrospray ionization-time of flight-mass spectroscopy. Zinc oxide is known to promote the formation of reactive.OH radicals and therefore is of special interest for the photocatalytic remediation of water from pharmaceuticals. Smaller particles are more active than larger ones due to increased specific surface area and adsorption. Interestingly, adsorption of the drug on ZnO is markedly improved at low concentration (<5 ppm), leading to the different degradation behaviors, compared to ppm concentrated solution. (C) 2014 Published by Elsevier B.V.