A theoretical model, based on numerical simulations, is provided to analyze the energy transfer in a two-dimensional mixed J-aggregate. The J-aggregate is modeled as a square arrangement of parallel donor molecules in which a given concentration of acceptors is randomly substituted. The Frenkel exciton states are calculated by numerical diagonalization of the aggregate Hamiltonian including the static disorder. The scattering rates are calculated using a model of exciton-phonon coupling. The concentration dependences of the absorption and fluorescence line shapes, the localization behavior of the exciton states and the time-resolved fluorescence decays (configurationally averaged) are obtained. The simulations are checked against recent experimental results on mixed J-aggregates of cyanine dyes. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim