The present research falls within the scope of the chemical influence of heavy metals on thermochemical conversion of biomass. Specifically, the impact of nickel during pyrolysis and CO2-gasification of willow samples was studied by experimental and thermodynamic approaches. For this purpose, willow wood samples were impregnated with various contents of nickel. The pyro-gasification tests were performed in a laboratory fixed bed reactor, providing the chemical composition of solid residue and major gases. Our thermodynamic simulations, based on a specific calculation procedure, were found to corroborate most of the experimental data. In addition to these methodological findings, the main results of the study are (i) Ni has a noticeable catalytic activity even at low contents (between 0.016 and 0.086 mol per kg of wood), (ii) due to the low oxygen potential of the system C + CO2, Ni stays in an active metal form as long as some carbon is left in the reactor, and (iii) Ni prevents the volatilization of sulfur during the gasification, due to the formation of a stable Ni3S2 solid phase.