Optoacoustic (photoacoustic) imaging assumes that the detected signal varies linearly with laser energy. However, nonlinear intensity responses as a function of light fluence have been suggested in optoacoustic microscopy, that is, within the first millimeter of tissue. In this study, we explore the presence of nonlinearity deeper in tissue (similar to 4 mm), as it relates to optoacoustic mesoscopy, and investigate the fluence required to delineate a switch from linear to nonlinear behavior. Optoacoustic signal nonlinearity is studied for different materials, different wavelengths and as a function of changes in the scattering and absorption coefficient of the medium imaged. We observe fluence thresholds in the mJ/cm(2) range and preliminary find that different materials may exhibit different nonlinearity patterns. We discuss the implications of nonlinearity in relation to image accuracy and quantification in optoacoustic tomography.