In this paper, postmortem aging investigations on insulation material of power transformers are presented. A methodology for taking paper samples in a systematical manner and a scheme for calculating an average degree of polymerization (DP) for the entire transformer out of the DP values of the individual paper samples will be shown. The typical DP profile in the axial direction of transformer coils will be explained using calculated distribution of losses within the coils of a 740-MVA generator stepup transformer. Further, correlations between the furan (2-FAL) concentration in the oil and the average DP of the investigated German GSU transformers and grid transformers are derived and compared with existing correlations.Index Terms-Concentration of furans in the oil, degree of polymerization, paper degradation, postmortem aging investigation of power transformers.