The degree of polymerisation (DP) of electrical insulation paper is an invaluable tool for assessing its condition and is extensively used in laboratory tests to assess the life expectancy of transformer insulation. Accurate and reproducible determinations are essential if the results are to be used to calculate a meaningful life. Publication IEC 450 was prepared in 1974 as a standard for the measurement of the DP of new and aged electrical papers (the equivalent US standard is ASTM D4243). However, experience across a number of laboratories has indicated the need for further clarification in the experimental method, whichever standard is adopted. Errors arise during preparation of solutions, measurement of viscosity and calculation of DP, due to oxidative degradation of the solution, inconsistencies in the measurement temperature and the conversion of intrinsic viscosity to DP using the Mark Houwink Sakurada relationship. This paper quantifies errors associated with DP measurement which are not described in existing standards.