The nonlinear absorption properties of two series of novel free base and metalated meso 5,10-A 2 B 2 sub- stituted porphyrins, both bearing p -tolyl as an A substituent and TMS-ethynyl or bromine as a B substitu- ent, were investigated with the open Z-scan technique at 532 nm in the ns time regime. Most of the compounds exhibited a transmission drop with increasing input fl uence. This behavior is desirable for their applications in optical limiting. More complex responses: a drop in transmission followed by an increase in transmission or an increase in transmission followed by a transmission drop, with increasing input fl uence, were detected for certain compounds. All of the recorded responses were successfully fi tted with a four-level model with simultaneous two-photon absorption arising from the higher excited states (consecutive one- + one- + two-photon absorption). The TMS-ethynyl group was found to be a more e ffi cient meso substituent in optical limiting than the bromine atom. Indium, lead and zinc com- plexes with TMS-ethynyl substituents were the strongest positive nonlinear absorbers amongst com- pounds studied which makes them the most interesting candidates for optical limiting application. ; PUBLISHED ; This work was supported by a grant from Science Foundation Ireland (SFI P.I. 09/IN.1/B2650). J.W. acknowledges the financial support from the 100-Talent Program of Chinese Academy of Sciences, the National Natural Science Foundation of China (NSFC, No. 61178007), and Science and Technology Commission of Shanghai Municipality (STCSM Nano Project, No. 11nm0502400, Pujiang Talent Program 12PJ1409400). The authors would like to thank Dr Eimear Finnigan for his help with the manuscript