We have recently demonstrated that carboxylic acid derivatized silver and gold colloidal particles can be immobilized through electrostatic interaction with fatty amine Langmuir monolayers at the air-water interface and that good quality multilayer films of the clusters can be deposited by the Langmuir-Blodgett (LB) technique. In this paper, we extend our study on the electrostically controlled immobilization process through an investigation of the partitioning of carboxylic acid derivatized clusters based on their size. Simultaneous immobilization of gold and silver clusters (130 ± 30 and 70 ± 12 Å diameter, respectively) from the same colloidal subphase at the air-water interface using octadecylamine Langmuir monolayers has been followed by π-A isotherm measurements as well as optical absorption, quartz crystal microgravimetry (QCM), and X-ray fluorescence (XRF) analysis of the built-up LB films. Even for small concentrations of gold relative to silver clusters in the subphase, the density of the larger gold clusters in the LB films was found to be surprisingly high in relation to the smaller silver clusters. While the π-A isotherms stabilized within 3 h of spreading the fatty amine monolayer, the cluster density at the air-water interface continued to increase, taking up to 12 h to reach equilibrium. The larger gold clusters reached an equilibrium density at the interface much faster than the smaller silver clusters. The "reverse" fractionation of clusters observed is discussed in terms of an electrostatic model.