The utilization of fast pyrolysis condensates is hindered by unwanted properties like corrosivity, high viscosity and tendency to polymerization. These characteristics result from the high content of organic acids and multiple functional groups and C-C double bonds. As an alternative to severe hydroprocessing, the addition of alcohol and subsequent esterification of acids and acetalization of aldehydes is investigated. Pyrolysis condensates are mixed with butanol and an acidic catalyst is added. This mixture is heated and kept under reflux in a dean-stark-apparatus to continuously boil off the water. As reference homogeneous liquid acids are used and compared to solid acid catalysts like ZnO or zeolites. The advantage of the heterogeneous, solid catalysts is the possibility to remove the solids after reaction and use them again for another batch in contrast to the homogeneous acids, which remain in the solution and need to be neutralized at the end. The most promising catalyst candidates are then used in experiments with varying process parameters. All products are analyzed for total acid number, water mass fraction, dynamic viscosity and higher heating value. The total acid number can be reduced by 90 % (from 110 to below 10 mg g(-1)), the water mass fraction by 90 % (from 25 % to below 2 %), the viscosity by 70 % (from 18 to 5 mm(2) s(-1)) and the higher heating value can be increased by 45 % (from 16.6 to 31 MJ kg(-1)). These improvements should allow the utilization of upgraded pyrolysis liquids in standard boilers and as fuel in CHP plants. (C) 2017 Elsevier Ltd. All rights reserved.