Deactivation mechanisms of alumina-supported, Ni-based catalysts for tar reforming in biomass-derived syngas were evaluated using extended X-ray absorption One structure (EXAFS) spectroscopy. Catalysts were characterized before and after catalytic reaction cycles and regeneration procedures, which included oxidation by a mixture of steam and air, and reduction in hydrogen. Qualitative analysis of the EXAFS spectra revealed that oxidation of a portion of the Ni in the catalysts to form an oxide phase and/or it sulfide phase were likely scenarios that led to catalyst deactivation with time-on-stream and with increased reaction cycles, Deactivation through carbon deposition, phosphorus poisoning, or changes in particle size were deemed as unlikely causes. Quantitative analysis of the EXAFS spectra indicated sulfur poisoning occurred with time-on-stream, and the contaminating species could not be completely removed during the regeneration protocols. The results also verified that Ni-containing oxide phases (most likely a spinel also containing Mg and Al) formed and contributed to the deactivation. This study validates the need for developing catalyst systems that will protect Ni from sulfur poisoning and oxide formation at elevated reaction and regeneration temperatures.