In this work we have used the Photoacoustic Spectroscopy ( PAS) to determine in vitro the non-radiative relaxation time (NRRT) of a protoporphyrin IX (PpIX) standard solution and samples of PpIX( 1), PpIX( 2) and PpIX( 3) with Au nanoparticle concentrations of 0.001008, 0.00504 and 0.01008 mmol in 25 mL of water respectively. We have used PpIX disodium salt (DS) solution of 25% HCl. The results show that the NRRT average values, obtained for each one of the solution were: tau = 29 +/- 0.001, 84 +/- 0.001 and 62 +/- 0.009 ms for PpIX( 1), PpIX( 2) and PpIX( 3), respectively. These values were compared with some NRRT of triplet states reported in the literature for molecules with tetrapyrrolic structure, increasing the NRRT considerably. From each solution it was obtained its PAS signal phase as a function of the light modulation frequency from 17 to 80 Hz. UV-vis spectrophotometer, photoluminescence spectroscopy and Transmission Electron Microscopy (TEM) were used in order to obtain the optical absorption spectra, the photoluminescence intensities, and the gold nanoparticle sizes respectively. Our investigations are devoted to improve the thermal treatments of drugs the porphyrins as photosensitizers used in image photodynamic therapy. (c) 2008 Elsevier B. V. All rights reserved.