The objective of this research project was to test the hypothesis that separation of char with its associated mineral matter from pyrolysis vapors before condensation will lead to improved bio-oil quality and stability with respect to storage and transportation. The metric prescribed by the U.S. Department of Energy (DOE) to evaluate stability in this case was a 10-fold reduction in the rate of increase of viscosity as determined by an accelerated aging test. The primary unit operation that was investigated for this purpose was hot gas filtration. A custom-built heated candle filter system was fabricated by the Pall Corporation and furnished to the National Renewable Energy Laboratory (NREL) for this test campaign. This system consisted of a candle filter element in a containment vessel surrounded by heating elements on the external surface of the vessel. The filter element and housing were interfaced to NREL's existing 0.5 MTD pyrolysis process development unit (PDU). For these tests, the pyrolysis reactor of the PDU was operated in the entrained-flow mode. The hot gas filter (HGF) test stand was installed on a slipstream from the PDU, so that both hot gas filtered oil and bio-oil that was not hot gas filtered could be collected for purposes of comparison. Two filter elements from Pall Corporation were tested: (1) porous sintered stainless-steel (PSS) metal powder and (2) sintered ceramic powder. A sophisticated bio-oil condensation and collection system was designed and fabricated at NREL and interfaced to the slipstream filter unit. The test campaign on vapor-phase filtration of biomass-derived pyrolysis oil demonstrated that a bio-oil with substantially improved properties can be obtained by application of hot gas filtration. The ceramic filter element and test stand supplied by Pall Corporation and the vapor condensation and collection system designed and fabricated by NREL both demonstrated very good operability. Application of periodic blowback was shown to be effective in maintaining the filter element pressure drop within acceptable limits, and filter plugging was never experienced. A bio-oil with greatly reduced alkali and alkaline earth metals and very low solids content was produced. Bio-oil obtained by hot gas filtration with a PSS element had elevated iron content, suggesting that the material of construction is not suitable for this application. The PSS-filtered bio-oil also did not pass the viscosity metric of a 10-fold reduction in the rate of viscosity increase as determined by the accelerated aging test at 80 degrees C. Bio-oil obtained by hot gas filtration with a ceramic (Dia-Schumalith sintered ceramic powder) filter element was also low in alkali and alkaline earth metals and total solids and did not exhibit high iron content. The ceramic-filtered oil passed the viscosity metric, indicating that this oil should be much improved with respect to storage and transport stability. Total mass loss because of hot gas filtration was estimated to be in the range of 10-30% by weight.