Incidental NPs are produced and dispersed in the environment in much larger amounts than manufactured NPs and with much less consideration of their environmental impact. This is potentially the case for ZnS NPs that form in a variety of anaerobic environments such as municipal and agricultural organic wastes. As an example, ZnS NPs with sizes ranging from 2.5 to 7.5 nm in diameter were observed in final sewage sludge products potentially accounting for about 80% of the Zn present in the sludge.1 In most countries, these organic wastes are spread on cultivated soils as fertilizer, which may represent an important source of nanomaterial into the environment. Zinc can be phytotoxic for crops at doses commonly found in organic wastes.2 Although ZnS is poorly soluble from a thermodynamic point of view (Ksp=10-24.7), ZnS NPs are known to transform to oxidized products in oxic environments quicker than bulk ZnS. As an example, the oxidation rate of bulk sphalerite was shown to be relatively slow when incubated in soils under oxic conditions: 0.6% per year in a sub-tropical soil and 1.2 % per year in a temperate soil.3 In contrast, we have recently shown that ZnS NPs formed in pig slurry were quickly oxidized after spreading on crop fields.4 The parameters controlling the oxidation of ZnS NPs are currently unknown. Available data is insufficient to elucidate whether the reported differences in oxidation rate are related to medium properties or to size effects. Given the likely dominance of ZnS NPs in municipal and agricultural wastes and the widespread use of these wastes in agricultural practices, a more fundamental understanding on the factors controlling the formation, chemical reactivity, mobility, bioavailability and toxicity of ZnS NPs under different environmental conditions is needed. Here, we will present some results regarding the characterization of ZnS NPs in different organic wastes (e.g. pig slurry, sewage sludge) as well as preliminary results on the sizedepe