The stability and reactivity of five different thermostable fungal laccases from the species Trametes hirsuta, Melanocarpus albomyces, Thielavia arenaria (two laccases) and Chaetomium thermophilum were investigated in the presence of organic solvents. Oxidations of small organic phenolic compounds, matairesinol and 7-hydroxymatairesinol lignans, as well as synthetic lignin dehydrogenation polymer DHP in aqueous solutions of ethanol and propylene glycol solvents were investigated using analysis of oxidation rates, high performance liquid chromatography and size-exclusion chromatography. The laccases showed variability in their solvent tolerance. The redox potential of the laccases appeared not to be the main factor determining the efficiency of the polymerization reactions of complex phenolic model compounds in aqueous organic solutions. Nuclear magnetic resonance spectroscopic analysis of laccase treated DHP in 50% propylene glycol indicated that the formation of new biphenylic 5–5′ structures was favored in laccase-catalyzed radical coupling reactions over the other possible reactions through the phenolic groups forming new 5—O—4 ether bonds. The polymerization reactions took place even at high concentrations of solvents, which already inhibited the enzyme activity, encouraging enzymatic upgrading of lignin in organic solvents to be studied further. Thus, it was confirmed that thermostable laccases are potential enzymes for various industrial applications where organic solvents are required for the reaction systems.