Sizes of aquatic colloidal nanoparticles are determined by utilizing the laser-induced breakdown detection technique. Specifically, the number of breakdown events is measured as a function of area in laser-induced plasma images, generated from a mixture of two different size nanoparticles in aqueous solution, by minimizing the energy of the incident laser irradiation to generate the plasma. We find that the accuracy of measuring the sizes of nanoparticles in mixtures is greatly improved when selecting the plasma images produced only within one-half of the Rayleigh range of the focused Gaussian beam where the irradiation is most uniformly distributed. (C) 2008 Elsevier B.V. All rights reserved.