A previously established treatment for photochemical processes in the presence of one receptor has been extended for the case of solutions containing mixtures of two (or more) receptors. The treatment gives variations in emission intensities and Stern–Volmer constants when the concentrations of receptors are changed. The quenching of 1-pyrene-carboxaldehyde by iodide ion in mixtures of β- and 2-hydroxypropyl- β- cyclodextrins has been used to check the new treatment. These mixtures are of three classes. Mixtures of class (i) are a model of a receptor containing two kinds of binding sites with different affinities towards a ligand. The second class of mixtures, (ii), are models for multisite receptors containing only one type of site. These sites however can change their affinity for a given ligand as a consequence of external stimuli, such as a change in pH. Finally, mixtures of class (iii) would be representative of polymers with two different binding sites in which the occupation of one of them produces, through a configurational change, the appearance of the previously hidden second class of sites.