Quantification of extrinsically administered contrast agents in optoacoustic (photoacoustic) tomography is a challenging task, mainly due to spectrally-dependent contributions from absorbing background tissue chromophores leading to strong changes in the light fluence for different positions and wavelengths. Herein we present a procedure capable of self-calibrating light fluence variations for quantitative imaging of the distribution of photo-absorbing agents. The method makes use of a logarithmic representation of the images taken at different wavelengths assisted with a blind unmixing approach. It is shown that the serial expansion of the logarithm of an image contains a term representing the ratio between absorption of the probe of interest and other background components. Provided the background variations are not very high, this term can be isolated with an unmixing algorithm, so that the concentration of the probe can subsequently be resolved.