Zinc oxide nanoparticles (ZnO NPs) are amongst the most commercialized engineered nanomaterials. Their biological impact in aquatic organisms has been associated to dissolution, but there is also evidence of nano-specific effects. In this study the waterborne uptake and efflux kinetics of isotopically labelled 68ZnO NPs (7.8 ± 1.2 nm), in comparison to aqueous 68Zn and 68ZnO bulk particles (up to 2 μm), were determined for the estuarine snail Peringia ulvae following a 7 d exposure (nominally 20 µg 68Zn L-1) and 28 d depuration. Detection of the 68Zn label was achieved by high precision multiple-collector ICP-MS (MC-ICP-MS). Previous characterization in artificial estuarine water revealed that the NPs underwent initial aggregation, and solubilized up to 60% within 1-2 days. Bulk and aqueous forms were significantly more bioavailable than 68ZnO NPs (p<0.05), but after correcting for dissolution, aqueous (0.074 L-1 g-1 d-1) and NP (0.070 L-1 g-1 d-1) uptake rate constants were highly comparable. The rate constant of loss for 68Zn aqueous (0.012 ± 0.005 d-1) and 68ZnO NPs (0.012 ± 0.007 d-1) were identical. These results strongly suggest that in this exposure scenario the bioaccumulation of Zn from ZnO NPs is primarily dependent upon solubility.