Production of hydrogen (H-2) from catalytic steam reforming of the aqueous phase of bio-oil was investigated in a fixed bed tubular flow reactor over nickel-based alumina-supported catalysts promoted with magnesia (Ni-MgO/Al2O3). The effects of time, amount of Ni, preparation condition, and initial bio-oil to water ratio on the yield of various outlet gases including hydrogen was investigated at 850 degrees C, and the outlet gas distributions were obtained. The average H-2 yield was very low with a maximum of 30% over the alumina support when the aqueous phase of the bio-oil at a bio-oil to water ratio of 1 was employed. The hydrogen yield nearly doubled with the addition of 12.8% nickel and 33.3% magnesia for the three bio-oil aqueous phase samples at various bio-oil to water ratios. This effect was more pronounced in the aqueous bio-oil phases with greater water content. On the contrary, the effect of the preparation method on H-2 yield was more pronounced in the aqueous phase samples with lower water content. Among the catalysts tested, the highest H-2 yield (61%) was achieved over Ni-MgO/Al2O3-3 with the aqueous phase of bio-oil with a bio-oil to water ratio of 1/1, indicating that a greater bio-oil to water ratio does not necessarily provide greater H-2 yield.