5,15-Bis(pyrimidin-2-ylthio)porphyrins have been synthesized. Their electrochemical oxidation leads to the formation of mono- and bis-C-N-fused thiopyrimidinium intermediates depending on the applied charge and potential. These latter undergo nucleophilic attack with water during workup that drives the ring opening of the pyrimidinium moiety. When piperidine is added before or after workup, the neutral fused porphyrinthiazin-2-amines are generated, and they exhibit a significant bathochromic shift of their Soret and Q bands.