Engineered metal and metal oxide nanoparticles (NPs) will change in size and surface properties and are expected to agglomerate and partly dissolve, when exposed to the aquatic environment. Toxic effects of some metal ions (e.g. Ag⁺, Cd2+, Cu²⁺, Zn²⁺, etc.) are very well documented. Some nanoparticulate forms of those metals show similar characteristics (e.g. Ag nanoparticles), but it is still not clear whether recorded toxic effects are caused by the particulate or dissolved form, or both. That is why it is of extreme importance to accurately measure dissolved metal and metal oxide nanoparticle fractions in different aquatic conditions. For this study we have selected metal and metal oxide NPs which are increasingly incorporated into a wide range of products and applications: Ag ( 9 coatings), ZnO (2 sizes), Cu (carbon coated; 2 sizes) and CuO (2 sizes). All experiments were performed in an artificial solution (algal growth media) which represents natural water, at 23 0C and no light conditions for a maximum of 19 days. In the experiments with AgNPs the pH was 7.5 and in the experiments with ZnO, Cu and CuO NPs two different pHs were used: 7.6 and 6.1. The dissolved fraction of metals from nanoparticles in aqueous media was measured using DGT (Diffusion Gradients in Thin films) technique. The results were compared with other available techniques, such as dialysis membrane (DM) and ultrafiltration (UF). Ag NPs size, surface charge and particle dissolution was coating dependent and constant throughout the experiment. Good correlation was achieved among the three techniques used for dissolved silver measurements, but DGT measured 2-5 times less Ag. Likely reasons are: difference in pore size, difference in sampling time, release of Ag⁺ weakly adsorbed on particle surface by UF and DM and/or difference in diffusion of organic Ag complexes. A small part of ZnO NPs agglomerated very fast after the start of the experiment and disappeared from the aqueous media due to sedimentation, or adsorption on the bottles wall. ZnO NPs left in the aqueous media dissolved almost completely, within hours, after the start of the experiment, especially at lower pH (6.1), as indicated by both DGT and ultrafiltration, for both sizes. Most of carbon coated Cu-NPs disappeared from the aqueous media due to agglomeration and sedimentation, especially smaller NPs at lower pH. When compared to total Cu concentrations present in aqueous media, the larger Cu particles dissolved almost completely at lower pH (6.1) within 2 days. Same particles dissolved around 50 % (measured by DGT and UF) at more neutral pH (7.7). The smaller Cu-NP behaved similarly. Significant increase in Cu NPs dissolution with experimental time was recorded for bigger particles at lower pH, only. Significant amount of CuO NPs agglomerated and disappeared from the aqueous solution, except smaller CuO NPs at lower pH. Smaller CuO NPs dissolved almost completely at lower pH, as well as a significant fraction of the larger CuO-NP At higher pH the dissolution was significantly lower (about 10% of the smaller and 1 % of the larger CuO-NP). In comparison to the total Cu amount introduced in aqueous media (0.8 mg/L of Cu from 1 mg/L CuO NPs), there is still significant amount of dissolved copper (50-70% measured by DGT and UF) at lower pH, but very small amount (less than 1%) of dissolved Cu at higher pH after 2 days. DGT and UF techniques produced similar results for Zn and Cu measurements, but DM measured significantly lower values, probably because of Zn²⁺ and Cu²⁺ adsorption onto the membrane. The DGT technique performed very well for the measurement of dissolved ions in presence of nanoparticles, but more testing is needed, especially in more complex, natural water composition with natural organic matter.