Pyrene binds to γ-cyclodextrin and forms host–guest complexes with different stoichiometries. The fluorescence of pyrene was employed as a qualitative signature for these different complexes. Solutions prepared at room temperature were shown not to be in thermodynamic equilibrium, since a heating cycle of 20 min at 80 °C followed by cooling led to changes in the relative concentrations of the pyrene–γ-cyclodextrin complexes. This temperature-annealing effect has consequences when determining thermodynamic parameters such as equilibrium constants and host–guest stoichiometries. Key words: cyclodextrin, pyrene, aggregates, fluorescence.