With the development of material sciences, the characterization of nanomaterials has become a critical issue in managing their fascinating size-dependent physical and chemical properties. Controlling these properties from the synthesis to the application phase, and consequently to their fate as a worldwide environmental and societal concern is becoming more and more imperative. The potential toxicity of nanoparticles needs to be evaluated when developing applications in a responsible way. Zinc oxide ZnO nanoparticles can be found largely as powders and dispersions with antibacterial, anti-corrosive, antifungal and UV filtering properties. ZnO nanoparticles can also be used for various applications ranging from food and cosmetics up to coatings agent and in the manufacturing of concrete. Research is actively being conducted towards in solar cells, photocatalysis, optical devices and sensors which have already started to show economic potential worldwide. The principal techniques currently used to achieve the characterization of nanoparticles are physical and physico-chemical methods, such as Transmission Electron Microscopy (TEM), X-ray diffraction, and optical spectroscopies. All these analytical tools are excellent for global analyses of clusters and nanomaterials. Soft ionization mass spectrometry (MS) methods such as Matrix Assisted Laser Desorption Ionization coupled with Time of Flight MS (MALDI-TOFMS) have already proven their potential as tools in the nanometrology of small-sized II-VI quantum dots (QDs) such as CdS, CdSe, ZnS and ZnSe. Mass spectra of these nanocrystals are consistent with TEM and optical spectroscopy measurements [1-2]. In this paper, we present a joint physical/physico-chemical study and, more specifically, the first application of MALDI-TOF-MS to analyze small-sized ZnO QDs (3-3.5 nm diameter range) synthesized by sol-gel chemistry and stabilized through an aminosilane coating. The organic shell increases the QDs stability and dispersibility in aqueous solution. The ligands were first quantified by thermogravimetric analysis (TGA), then a careful investigation of the stability of ZnO QDs was initiated once these QDs were dispersed in different media (water, biological buffer,ï¿½) for a period up to 6 weeks. Positive ion mode mass spectra showed a decrease in mass and consequently in diameter during aging, which can be ascribed to the degradation of ZnO QDs. In conclusion, the unique combination of MALDI-TOF-MS and physico-chemical techniques brings new insights concerning the structure analysis, the stability and consequently the potential toxicity of ZnO QDs. This new strategy in nanometrology will be extended to other II-VI materials in the near future.  A. Aboulaich, D. Billaud, M. Abyan, L. Balan, J.J. Gaumet, G. Medjahdi, J. Ghanbaja, R. Schneider ACS Appl. Mater. Interfaces, 4 (5) 2561-2569 (2012).  M. Fregnaux, J.J. Gaumet, S. Dalmasso, J.P. Laurenti, R. Schneider Microelectron. Eng. 108, 187-191 (2013).