Optical limiting (OL) performances of two trimeric metallo-phthalocyanines namely, 2,4,6-tris[2-oxa-9,10,16,17,23,24-hexa(hexylthio) phthalocyaninato M(II)]-s-triazine (M = Zn for compound ZnPc and Cu. for compound CuPc) were investigated by using the Z-scan technique. The measurements were performed using collimated 4 ns pulses generated from a frequency-doubled Nd:YAG laser at 532 nm wavelength. OL parameters of the ratio of the excited state to ground state absorption cross-sections K, the effective non-linear absorption coefficient beta(eff), the linear absorption coefficient alpha(0), limiting threshold F-th and the saturation density or energy density F-sat values were determined for the investigated compounds. The results indicated that both compounds exhibited good OL performances. ZnPc shows slightly better OL parameters than that of CuPc. (c) 2008 Elsevier B.V. All rights reserved.