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Rarely observed nanoparticle dissolution rate data have been collected and explained for an environmentally and industrially relevant nanomaterial (PbS, the mineral galena) as a function of its particle size and aggregation state using high-resolution transmission electron microscopy (HRTEM) and solution analysis. Under identical anoxic acidic conditions (pH 3 HCl), it has been determined that the dissolution rate of PbS galena varies by at least 1 order of magnitude simply as a function of particle size, and also due to the aggregation state of the particles (dissolution rates measured are 4.4 × 10⁻⁹ mol m⁻² s⁻¹ for dispersed 14 nm nanocrystals; 7.7 × 10⁻¹⁰ mol m⁻² s⁻¹ for dispersed 3.1 μm microcrystals; and 4.7 × 10⁻¹⁰ mol m⁻² s⁻¹ for aggregated 14 nm nanocrystals). The dissolution rate difference between galena microparticles and nanoparticles is due to differences in nanotopography and the …