Aggregation equilibrium and the fluorescence properties of hydroxoaluminium tricarboxymonoamidephthalocyanine adsorbed on microgranular cellulose have been studied at different dye loadings. Up to a concentration of nearly 3 × 10⁻⁶ mol phthalocyanine (g cellulose)⁻¹ diffuse reflectance spectra may be interpreted on the basis of a simple monomer–dimer equilibrium. Monomer and dimer spectra are similar to the spectra of the monomeric dye in solution. The solid-state dimer spectrum is red-shifted with respect to that of the monomer and this is attributed to the coplanarity of dimers. Fluorescence spectra and quantum yields show typical effects of re-absorption and re-emission of light. In particular, the observed fluorescence quantum yields depend on concentration and span the range 0.29–0.07. To account for these effects as well as the effect of aggregation on fluorescence quantum yields and to obtain corrected fluorescence spectra a model based on the Kubelka–Munk theory of diffuse reflectance is developed. The application of this model to the case under study yields a true fluorescence quantum yield ϕ = 0.46 ± 0.02 in the whole range of concentrations, which is slightly higher than the value found for the same dye in dimethyl sulfoxide solution.