The aggregation of a copper(II) tetrasulfonated phthalocyanine (CuPcS4) dye in aqueous sodium chloride solution has been studied using UV-vis spectroscopy and statistical mechanics. The concentration dependences of the molar absorption coefficients at 626 and 665 nm have been measured for CuPcS4 solutions with concentrations in the range 10(-7) to 10(-3) M and salt concentrations between 0 and 2 M. It is commonly believed that such results can be fitted adequately by assuming that, a monomer-dimer equilibrium operates in the solution, but it is shown that this model has serious deficiencies, particularly at low salt concentrations. The experimental results have been analyzed using new expressions derived using statistical mechanics, taking into account the formation of trimers and higher aggregates. The model is based on the fact that pi-pi interactions between the essentially planar phthalocyanine molecules favor the formation of columnar aggregates. Therefore, cluster partition functions can be approximated in terms of two parameters corresponding to molecules with one and two nearest neighbors within the stack. The resulting theoretical expressions are shown to provide excellent fits to the experimental results over the entire range of dye and salt concentrations considered. The fitted equilibrium constants using the. monomer-dimer equilibrium indicate an increase in aggregation with increasing salt concentration. In contrast, the fitted equilibrium constants from the new theory indicate that aggregation is suppressed at high salt concentrations.