The effect of external oxygen supply on laccase mediated oxidation of different types of lignins [Organo-solv, lndulin AT, sodium (Na) lignosulfonates and magnesium (Mg) lignosulfonates] was investigated. This study demonstrates for the first time that oxygen as an essential electron acceptor is limiting in laccase mediated oxidation of lignin processes. Oxygen levels decreased to undetectable levels and remained undetectable for a long period in shaking samples without external oxygen supply. The decrease in fluorescence intensity attributed to the modification in conjugated carbonyl, biphenyl, phenylcoumarins and stilbene groups during laccase oxidationwas 40 times faster in sample mixtures supplied with pure oxygen than shaking samples without external oxygen supply and has direct implications on saving time. However, despite a general increase in molecular weight in all laccase treated lignins, pure oxygen supply does not always lead to the highest Mw. For example Mg-lignosulfonates treated with Trametes villosa laccase under shaking conditions without external oxygen supply had the highest Mw 85,777 kDa as compared to 68,842 kDa in the presence of pure oxygen. This study therefore shows demonstrates the importance of supplying oxygen to laccase mediated reactions in order to shorten the incubation time and ensure complete oxidation. (C) 2015 Elsevier Ltd. All rights reserved.