We have investigated the transient optical bleaching of two sterically stabilized kerosene based magnetite ferrofluids and of a dye solution (quinizarin in toluene). Master plots are constructed from model calculations of the initial rise of the transmitted intensity, which clearly prove that the bleaching is caused by a thermophoretic migration of the absorbers out of the laser beam. The quantitative model allows for the determination of the thermal diffusion coefficients of the two ferrofluids investigated to 0.4 and 1.7x10(-8) cm(2)(s K)(-1), which agrees well with the literature value for a similar system. (C) 2003 American Institute of Physics.