The non-linear optical properties of metal-free phthalocyanine (MFPC) thin films were examined using the second harmonic at 532 nm from a pulsed Nd:YAG laser, and the continuous wave He-Ne and Ar+ lasers. The He-Ne laser transmission at fixed input intensity was found to increase temporally within a period of 12 h. The third-order non-linear susceptibilities (chi((3))) by four-wave mixing were measured for films of different thickness. The saturation intensity of MFPC, and its absorption cross-section, at 633 nm from a He-Ne laser, are reported. An optical bistability was recorded using a He-Ne laser. An AND logic gate was also demonstrated in the system. These phenomena in the system are attributed to refractive index modulation by thermal excitations.