Solely some degradation by-product of cellulose paper, detectable in oil may provide information on the condition of the paper without requiring invasive measures or de-energizing the transformer. In this contribution, oil impregnated Kraft paper samples were submitted to thermal aging with oxygen inlet in laboratory conditions. Many key parameters providing a 'picture' of the paper condition have been determined. From the obtained results, some correlations were found between the amounts of alcoholic chemical markers (methanol, ethanol) dissolved in oil and the degree of polymerization (DP v) together with the mechanical properties of Kraft papers. These correlations can be used as tools to assess the degradation of the solid insulation and ultimately to estimate the lifespan of power transformers. © 2014 IEEE.