Mineral oils are used in electric power transformers for their excellent dielectric properties. The continuous exposition to electrical discharges and thermal stresses may cause chemical adulteration in these substances which eventually could be cause of electrical catastrophic fails in the transformers. It is then important to evaluate periodically the quality of the oils to prevent major damages. The technique currently used for such purposes is gas chromatography, however its high inaccuracy iswell known. Chemical transformation of aged oils changes their thermal and optical properties and these could be used as alternative parameters for diagnostic purposes of electric power transformers. In this paper two configurations of photopyroelectric techniques were used for optical absorption coefficient (at 405 nm) and thermal diffusivity measurements of mineral oils aged up to 2300 h under controlled conditions. It was found that, at least up to this period of aging time, thermal diffusivity does not show significant differences, optical absorption coefficient at 405 nm, on the other hand, showed significant differences starting at 1000 h and then this optical parameter can potentially be used for diagnostic purposes.