The effect of calcium additives on the fast pyrolysis of switchgrass was studied by continuously pyrolyzing physical mixtures of the biomass with Ca(OH)(2), CaO, and Ca(COOH)(2) in a laboratory scale fluidized bed reactor. Initial tests were performed by cofeeding 220 g/h of switchgrass with Ca(OH)(2) at ratios of 0.4/1 and 0.8/1 Ca(OH)(2)/biomass, running at reactor temperatures of 500, 550, and 600 degrees C, and using nitrogen or recycled pyrolysis gas as the carrier gas. In comparison with control experiments (Ca-free, biomass only), cofeeding Ca(OH)(2) led to a decrease in the yield of both organic phase bio-oil and organic compounds solubilized in the aqueous phase, while noncondensable gas yields were increased. The bio-oils exhibited a reduced oxygen content, a lower concentration of highly oxygenated compounds such as acetic acid and levoglucosan, and a small increase in the concentration of phenols and hydrocarbons. When higher Ca/biomass ratios or higher temperatures were tested, bio-oil yields were further reduced while the bio-oil deoxygenation rate was only slightly higher. The input calcium salts were converted to CaCO3 because of a net trapping of CO2, promoting deoxygenation. Experiments with both N-2 and recycled pyrolysis gases as the carrier gas were performed to observe the effect of the changing atmosphere. The use of recycled pyrolysis gases led to increased bio-oil yields at temperatures of 500 and 550 degrees C, but a lower bio-oil yield at 600 degrees C for processing with Ca(OH)(2). Organic phase bio-oil carbon yields were 10.4, 17.4, 15.2, and 22.8% from biomass for Ca(OH)(2), CaO, Ca(COOH)(2), and the Ca-free control experiment, respectively, with oxygen contents of 21, 20, 19, and 29.7 wt % at 550 degrees C (600 degrees C for Ca-free control). The conversion of the input calcium salt to CaCO3 followed the pattern of Ca(OH)(2) > Ca(COOH)(2) > CaO, suggesting that bio-oil deoxygenation might not be only related to net CO2 trapping as CaCO3, but also to the catalytic activity of Ca2+.