This work focuses on the flow behavior and accelerated aging of pyrolysis oils produced in a screw reactor from different feedstocks. Beech wood, wheat straw, chicken manure, and sewage sludge were pyrolyzed at 350, 400, 450, and 500 degrees C, and the viscosity of the corresponding oils was measured at four different temperatures between 20 and 80 degrees C, with values up to 1 Pa s at 20 degrees C. Newtonian flow behavior was observed, except for wheat straw pyrolysis oils at temperatures below 50 degrees C. Absolute viscosity values as well as flow activation energies (EA), covering a range from 38 to 57 kJ mol(-1), turned out to be independent of the pyrolysis temperature, except for beech wood oil. In this latter case, a peak in viscosity and activation energy was found at a pyrolysis temperature of 450 degrees C and is attributed to the low water content of the oil obtained at this temperature. Accelerated aging experiments were conducted for 24 h at 40 and 80 degrees C. For the latter temperature, the effect of a free surface and an inert gas atmosphere on aging was examined. This enabled us to separate the effect of polymerization and evaporation of volatiles on the change in viscosity. In both cases, beech wood pyrolysis oil showed the strongest aging (similar to 10-fold increase in viscosity). The lowest aging was found for wheat straw pyrolysis oil in a free surface sample cell, while almost no aging occurred under an inert gas atmosphere. Therefore, we conclude that aging of beech wood oil is mostly due to polymerization, whereas aging of wheat straw oil is essentially due to evaporation of volatiles. For chicken manure and sewage sludge oils, both polymerization and evaporation contribute almost equally to the increase of viscosity when an open sample cell is used. For beech wood pyrolysis oil, size-exclusion chromatography revealed a change in molecular weight upon aging from M-W = 190 to 240 Da. A slight increase in M-W could be detected for chicken manure and sewage sludge, but no indication for polymerization was found for wheat straw. Gas chromatography coupled with mass spectrometry was used to determine the concentration of characteristic oil components quantitatively, confirming the conclusions from viscosity measurements regarding aging mechanisms.