Matrix-Assisted Laser Desorption/Ionization Time-of-Flight (MALDI-TOF) mass spectrometry was used to elucidate lignin-glyoxal reactions. The reaction of glyoxal with a low molecular weight lignin involves a number of different reactions; in particular lignin depolymerization and recombination through condensation reactions with glyoxal to form glyoxalene bridges linking both lignin units and mainly fractions of lignin units derived from degradation during depolymerization. The reactive hydroxyglyoxalated lignin is the one species still available for co-reaction for use in wood adhesives. The reactions described increase the viscosity of the lignin as the reaction proceeds towards higher molecular mass species and the condensed species grow in relative proportions. NMR analysis confirmed the existence of functional groups pertaining to structures that are formed from glyoxalation of the lignin observed by some results obtained by MALDI-TOF. MALDI-TOF mass spectrometry appears to be a suitable method for examining lignin-glyoxal reactions. (C) Koninklijke Brill NV, Leiden, 2012