Aggregation is a major factor affecting the photophysical properties of chlorophylls, For two [Pd]-bacteriochlorophyll derivatives that are currently under clinical testing as sensitizers for photodynamic therapy, aggregation control in aqueous solution has been studied with folded-sheet mesoporous silica (FSM) of different pore sizes (20, 45, 83 angstrom) and with detergent (Triton X-100). With both the moderately polar WST09 and the highly polar WST11, no pigment oligomers were formed in FSM, and the monomer-dimer equilibrium was shifted toward the monomer with decreasing pore diameter.