IT is known1–3 that the chlorophylls (but not their pheophytins) will aggregate in a variety of non-polar solvents (for example, benzene and carbon tetrachloride), with characteristic spectra for the respective monomers and dimers. These aggregates are concentration-dependent in the usual mass action sense up to a concentration of at least 10−2 moles/l. They may be deaggregated by solvents which co-ordinate with the magnesium, for example, alcohols, and the extent of deaggregation may be calculated from the characteristic spectra.