Femtosecond laser material processing as micromachining and nanoparticles fabrication require a careful control of the fluences deposited on the samples. In many cases, best results are obtained by using fluences slightly above the Laser Ablation Threshold (LAT), therefore its accurate determination is an important requirement. LAT can be obtained by measuring the intensity of the acoustic signal generated during the ablation process as a function of the laser fluence. In this work femtosecond laser ablation thresholds of commercially polished stainless steel plates, white high impact polystyrene, frosted glass, antique rag papers and silicon oxynitride thin films were determined by using laser ablation induced photoacoustics (LAIP). Results were compared with similar data previously obtained by using a nanosecond Nd:YAG laser.Centro de Investigaciones Ópticas