ZnO nanostructures have been prepared in distilled water by pulsed laser ablation, by varying the laser pulse energy between 20 and 150 mJ and for an ablation time of 10 min. The structural properties of the samples have been studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS), reflection electron energy loss (REELS), UV-vis and photoluminescence (PL) spectroscopies. The nanoparticles in colloidal suspension have shown a good degree of stoichiometry, as deduced by the analysis of the O 1s and Zn2p(3/2) photoemission spectra. The diffraction pattern and the interplanar spacings well match to the standard diffraction pattern of wurtzite zinc oxide (ZnO). The optical band gap calculated by the absorption spectra is found to be size dependent showing an increase for a decreasing particle size. As estimated from the analysis of TEM images and confirmed by the XRD data, the diameter of the nanocrystals within the greater nanostructures is mainly in the range of 1-10 nm. A large number of oxygen vacancies have been put in evidence by our PL measurements. This supports the lower density value of 6.12 g/cm(3), estimated by REELS in the free electron approximation, with respect to the one generally reported for thin film and crystal phases. (C) 2012 Elsevier B.V. All rights reserved.