A novel BaO-based sorbent is tested for in situ application as desulfurization agent in an allothermal biomass gasification process. BaO is stabilized against the formation of carbonate and thus theoretically allows desulfurization down to a few parts per million (volume) (ppmv) H2S even for high-steam and high-temperature conditions. In real process application, sintering of the sorbent occurred and had to be prevented by the addition of lime as separating agent. Additionally, a kinetic limitation of the desulfurization with BaO was shown for the in-bed sorption of sulfur. An increase of gasification temperature from 760 to 810 °C significantly improved the desulfurization performance. The observed behavior is in contrast to that of previously tested CaO-based in situ sorbents. CaO-based in situ desulfurization is limited to values of about 500 ppmv H2S for the conditions prevailing in the gasifier. With the novel BaO-based sorbent, a desulfurization from 85 to 35 ppmv residual H2S is shown upon addition of the sorbent to the gasifier. In experiments with a combined desulfurization of CaO and BaO, a release of H2S from CaS was shown once the H2S content in the gas drops below the CaO-based steady-state equilibrium of desulfurization. Particle analysis via SEM-EDX, XRD, ICP-OES, and BET complement the results of in situ desulfurization. SEM-EDX results indicate the stabilization of BaO in sintering bridges and a certain depletion of barium from the sorbent after in-bed application.