Energy and Fuels Vol.24 Nr.9, 5264-5272 ; This paper focuses on analytical methods to determine the composition of hydrotreated fast pyrolysis liquids. With this information, it is possible to gain insights in the chemical transformations taking place during catalytic hydrotreatment (hydrogenation and/or hydrodeoxygenation, HDO) of pyrolysis liquids. Three different samples, produced at different hydrotreatment severity levels (defined by temperature and residence time) using Ru/C as the catalyst, were analyzed in detail. The composition of the products was determined by solvent fractionation followed by detailed analysis of the various fractions by gas cheromatography/mass selective detector (GC/MSD), capillary electrophoresis (CE), and NMR (1H NMR, 13C NMR, and 31P NMR). The decrease in the carbohydrate fraction was easily followed by the Brix method after solvent fractionation.