The interaction of chloroaluminium phthalocyanine and aluminium porphyrin in solution was studied using UV-visible and fluorescence techniques. The spectra of the porphyrin alone exhibited new bands attributed to aggregates and a value of -12.6 U mol(-1) for the enthalpy of dimerization was obtained. A small dimerization constant of 5.8 x 10(3) M-1 was found for phthalocyanine homodimers and preferential formation of mixed dimers over porphyrin homodimers was observed experimentally and supported by molecular modelling. k(d) and Delta H values of 5.7 x 10(4) M-1 and -21.1 kJmol(-1) were found in the mixed system, along with an enhancement in phthalocyanine fluorescence. (c) 2005 Elsevier B.V. All rights reserved.