Both gas phase Na-n and Na-n(NaOH)(2) naroparticle ensembles (n = 50-250) are produced in a gas aggregation source and studied by using laser-ionization time-of-flight mass spectrometry. Heterogeneous Nan(NaOH)(2) clusters are shown to be produced by accretion of sodium atoms on nucleation centers initially formed by a chemical reaction between H2O and Na or Na-2 molecules. Mass distributions are compared for Na-n grown on an homogeneous nucleation center and for Na-n(NaOH)(2) developing on an heterogeneous seed in the same condensation source, allowing the comparison between homogeneous and heterogeneous clustering efficiency. (c) 2006 Elsevier B.V. All rights reserved.