In this paper a new experimental method for determining the kinetics of fast precipitation reactions is introduced. Use is made of a laminar jet reactor, which is also frequently applied to determine the kinetics of homogeneous gas-liquid reactions. The liquid containing one or more of the precipitating reactants passes a gas-filled reactor as a stagnant jet in which no mixing occurs. The remaining reactant needed for precipitation is supplied in gaseous form and causes the precipitation reaction to occur while it is diffusing into the jet. Hydrodynamics as well as transport phenomena are precisely known for this system, whereas agglomeration can be minimized by adjustment of the concentration of the solute supplied by the gas. The kinetics of the different crystallization steps can be determined by analyzing the size distribution of the produced particles. This new method is experimentally demonstrated for the precipitation of CuS using H2S gas. The obtained data were successfully used to simulate a packed bed absorber in which H2S is absorbed by a CuSO4 solution. (C) 2004 Elsevier Ltd. All rights reserved.