The excess polarizability volumes, Delta V-p of the relaxed S-1 and T-1 excited states of several covalently bridged zinc porphyrin dimers and their corresponding monomers have been measured using the flash photolysis time-resolved microwave conductivity technique. Delta V-p(S-1) increases from close to zero for a diaryl-substituted monomer up to a maximum value of 590 Angstrom(3) for a dimer coupled by a 9,10-diethynylanthracene bridge, yPyAyPy. The particularly large excess polarizability of yPyAyPy is attributed to strong electronic coupling between the porphyrin moieties resulting from stabilization of the cumulenic quinoidal resonance structure of the bridge. The strength of the electronic interaction, as indicated by the magnitude of Delta V-p(S-1), increases in the following order of bridging units: 1,4-phenylene < single sigma-bond < 1,4-diethynylbenzene < 2,5-diethynylthiophene approximate to butadiyne < 9,10-diethynylanthracene. The results provide an example of an inverse distance effect whereby the electronic interaction between porphyrin moieties actually increases with increasing length of the intervening bridge. The product of the intersystem crossing efficiency and the excess polarizability volume of the triplet state, phi(isc)Delta V-p(T-1), is more than an order of magnitude less than Delta V-p(S-1), indicating a much smaller degree of exciton delocalization in T-1 than in S-1. The microwave results are compared with results on the optical absorption and emission spectra which provide additional information on both the electronic and Coulombic excitonic interactions.