Results of experiments on laser-induced nucleation (LIN) in supersaturated (S = 1.20) aqueous ammonium chloride solutions are presented. Measurement of the particle-size distribution in unfiltered solutions near saturation (95%) indicates a population of nanometer-scale species with mean hydrodynamic diameter 750 nm, which is almost entirely removed by single-pass filtration through a poly(ether sulfone) membrane (0.2 m pores). Analysis of filter residues reveals iron and phosphate as major impurities in the solute. Experiments show that the number of nuclei induced by LIN can be reduced substantially by pre-processing (filtering or long-term exposure to laser pulses) and that this reduction can be reversed by intentional doping with iron-oxide (Fe 3 O 4) nanoparticles. The use of surfactant to assist dispersion of the nanoparticles was found to increase the number of laser-induced nuclei. We discuss the results with reference to mechanisms of nonphotochemical laser-induced nucleation (NPLIN).