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To go further in our assessment of methanol for the monitoring of the cellulosic insulation condition in electrical transformers, two of the paper/oil systems [one comprising a standard wood Kraft paper and the other, a thermally-upgraded Kraft paper (TU paper)] discussed in Parts 1 and 2 (Gilbert et al. in Cellulose 16:327-338. doi:, ; Cellulose 17:253-269. doi:, ) were reinvestigated under accelerated ageing conditions (T between 130 and 210 °C). The experimental data were tracked by testing two mathematical models, one that takes into account the inhomogeneous nature of the polymer by decreasing with time a pseudo-zero rate constant by a first-order process and the other, by simultaneously operating a separate first-order law for the three main categories of 1,4-β-glycosidic bonds in the polymer structure. These additional results provide evidence that the degradation proceeds via a random opening of these bonds in both the amorphous and crystalline regions, with the influence of a pyrolysis-like mechanism even at relatively low temperatures. Very good dependence with the temperature ( lnk vs 1/ T) is noted for the rate constants of the production of chain-end groups and the formation of methanol, confirming the importance of carrying out the ageing tests beyond the LODP of the specimens (leveling-off value of the degree of polymerization). The grouping of some of these data with those published in Part 2 for an identical system reveals that the degradation is governed by a single reaction pattern over 70-210 °C. The Arrhenius law applied to this data grouping shows an activation energy of 95 ± 4 kJ mol with a frequency factor (lnA) of 22 ± 1 for the depolymerization, while for the formation of CHOH, the respective values are 122 ± 6 kJ mol and 33 ± 2. Finally, the kinetic parameters obtained under accelerated ageing conditions over 150-210 °C show that the stabilizing substances of the TU-Kraft paper modify the bond opening mechanism by blocking the effect of the acid and water generated by the oil decomposition. When they are present in the fibrous structure, the activation energy of the reaction goes up to 148 ± 14 kJ mol with a frequency factor of 33 ± 4 for the depolymerization and 133 ± 11 kJ mol with a frequency factor of 33 ± 3 for the formation of CHOH. [ABSTRACT FROM AUTHOR]