Organosolv treatment is an environmentally friendly fractionation method that provides a stream of lignin with high purity. The severity of the treatment influences the chemical composition of the very complex lignin degradation product mixtures produced. Understanding the effects of the severity of the process on the types and relative abundances of the individual compounds in the mixtures is of great importance in optimizing the treatment and developing downstream conversion processes. In this study, seven Organosolv switchgrass lignin samples, prepared at different reaction temperatures and using different acid concentrations and reaction times, were initially analyzed using high resolution mass spectrometry with electrospray ionization in negative ion mode ((-) ESI HRMS). Fast pyrolysis combined with negative atmospheric pressure chemical ionization in subion mode and HRMS (py / (-) APCI HRMS) was also used to characterize the lignin degradation products. This method produced data similar to (-) ESI HRMS. It has been found that lignin monomers and dimers constitute the majority of the compounds in the mixtures. High-performance liquid chromatography coupled with (-) ESI high-resolution multistage tandem mass spectrometry (HPLC / (-) ESI HRMSn) based on collision activated dissociation (CAD) was used to obtain structural information for the most abundant compounds as well as a β-O-4 dimer with a relatively low abundance in the Organosolv lignin samples. The relative abundance of lignin-carbohydrate complexes was high under mild Organosolv treatment conditions, but low under moderate and severe treatment conditions. Since lignin compounds with β-O-4 bonds are not stable under acidic conditions, it has been found that the relative abundances of these compounds are very low. The relative amounts of lignin monomers decreased with increasing severity of the treatment, while the relative abundances of lignin dimers, trimers and larger oligomers increased, possibly due to repolymerization reactions under the more severe treatment conditions.