We observe a dramatic enhancement of simultaneous two-photon absorption (TPA) in a series of substituted tetraazaporphyrins with multiple electron-accepting groups and show for the first time that the TPA cross-section increases in linear proportion with the substituent's Hammett constant. The measured cross-section amounts to sigma(2) = 1.6 x 10(-47) cm(4) s/photon for octakis(4-nitrophenyl)-tetraazaporphine at 770 nm. This large value is explained by both the resonance enhancement by a strong Q(x)(0-0) linear transition and the presence in the same spectral region of a strong two-photon allowed gerade-gerade transition. (C) 2002 Elsevier Science B.V. All rights reserved.