A new method for the determination of coagulation rate constants for monodisperse, neutral particles is described. In this method, a differential mobility analyzer (DMA) is used to prepare a monodisperse aerosol and a second DMA is used to separate the coagulation products from the original monodisperse particles. The experiments are carried out under initial rate conditions so that typically 5-9% of the monomer particles undergo coagulation. Experimental results at 298 +/-1 K for H2SO4/H2O particles with diameters of 49-127 nm and a composition of 72-73% H2SO4 by mass gave enhancement factors, relative to rate constants calculated for hard spheres, that vary from about 1.2 for the largest particles to 2.8 for the smallest particles. Fitting these results to a theoretical expression accounting for van der Waals forces gives a Hamaker constant of (6.4 +/- 2.6)x 10(-13) erg. We also give convenient formulas for computing coagulation enhancement factors from the Hamaker constant. (C) 2001 Elsevier Science Ltd. All rights reserved.