Electron autodetachment from isolated metal phthalocyanine-tetrasulfonate tetraanions (MPc(SO3)(4)(4-)) (M=Ni, Cu) was studied at room temperature in an FT-ICR mass spectrometer under UHV conditions. For M = Cu, the electron loss rate was measured for the as-prepared isomer mixture. In the case of M = Ni, two chromatographically purified fractions containing different compositions of constitutional isomers were studied. The observed rate constants for these fractions differed by a factor of 37 +/- 5. Photoelectron spectroscopy indicates that both Cu and Ni compounds comprise isomer ensembles which are electronically metastable. Model calculations suggest that the differing electron loss rates observed for the Ni isomer fractions reflect tunneling through repulsive Coulomb barrier surfaces, which are themselves strongly isomer dependent in shape and height.