Engineered metal and metal oxide nanoparticles (NPs) are expected to agglomerate and partly dissolve, when introduced into aquatic environments. Toxic effects of some metal ions (e.g. Ag⁺, Cu²⁺, Zn²⁺) are very well documented. The observed toxic effects of nanoparticles may be caused by the particulate or dissolved form, or both. In this project, the dissolution of some widely used nanoparticles (Ag, ZnO, Cu-carbon coated and CuO) has been studied over days to weeks in natural water samples. The dissolved fraction of metals from nanoparticles was determined using DGT (Diffusion Gradients in Thin films), dialysis membrane (DM) and ultrafiltration (UF). The natural waters from a wastewater treatment plant effluent (WWTP Dübendorf), from the lakes Greifen, Lucerne and Cristallina (TI) greatly differed in pH, major ion concentrations and dissolved organic carbon (DOC). The dissolution was highly dependent on nanoparticle type and natural water composition. Relatively small amounts of Ag-NPs dissolved during the experimental period of 9 days. Dissolved Ag concentrations were below 1% of total Ag in water from the WWTP Dübendorf and from lakes Greifen and Lucerne, but higher with 3.7 % in water from Cristallina lake with lower pH 6.4. In contrast, most of ZnO-NPs present in suspension dissolved very fast (within first few hours) and almost completely, especially in the waters with lower pH (Cristallina lake). CuO-NPs also dissolved faster and more completely in water from Lake Cristallina. At the end of experimental period, most of Cu present in the suspension was in dissolved form. In contrast, Cu/C-NPs behaved similary in all four different natural waters and were removed from suspension, probably because of their hydrophobic characteristics, which cause agglomeration, adsorption and sedimentation. From the rest of Cu/C-NPs present in the suspensions (10%), around half dissolved during first two days. At the end of experimental period (9 days), only 2-5 % of total Cu introduced in the natural waters was in dissolved form. When the techniques for dissolved metal measurements are compared, clearly DGT and UF perform well in all types of natural waters (investigated until now), whereas DM gave only in few cases comparable results to the other methods These results indicate that NP dissolution may be an important process under conditions of natural waters or of wastewaters.