Bio-oil reforming is considered for syngas or H 2 production. In this work, we studied the steam reforming (SR) of two raw bio-oils without adding external steam, using a recently-developed catalyst, Ni-UGSO. Experiments were performed at temperature (T) = 750-850 • C and weight hourly space velocity (WHSV) = 1.7-7.1 g/g cat /h to assess C conversion (X C ) and product yields. The results show that, in all conditions and with both bio-oils tested, the catalyst is stable for the entire duration of the tests (~500 min) even when some C deposition occurred and that only at the highest WHSV tested there is a slight deactivation. In all tests, catalytic activity remained constant after a first, short, transient state, which corresponded to catalyst activation. The highest yields and conversions, with Y H 2 , Y CO and X C of 94%, 84% and 100%, respectively, were observed at temperatures above 800 • C and WHSV = 1.7 g/g cat /h. The amount of H 2 O in the bio-oils had a non-negligible effect on catalyst activity, impacting Y H 2 , Y CO and X C values. It was observed that, above a critical amount of H 2 O, the catalyst was not fully activated. However, higher H 2 O content led to the reduction of C deposits as well as lower Y H 2 and Y CO and, through the water-gas-shift reaction, to higher Y CO 2 (CO 2 selectivity). Fresh and spent catalysts were analyzed by physisorption (BET), X-ray diffraction, scanning electron microscopy and thermogravimetric analysis: the results reveal that, during the oils' SR reaction, the initial spinel (Ni-Fe-Mg-Al) structures decreased over time-on-stream (TOS), while metallic Ni, Fe and their alloy phases appeared. Although significant sintering was observed in used catalysts, especially at high H 2 O/C ratio, the catalyst's specific surface generally increased; the latter was attributed to the presence of nanometric metallic Ni and Ni-Fe alloy particles formed by reduction reactions. A small amount of C (4%) was formed at low H 2 O/C.