Ni-Co/Mg(Al)O catalysts with different Ni-Co ratios (40-0, 30-10, 20-20, 10-30 and 0-40 wt%) have been tested for steam reforming of hydrocarbon impurities in a model biomass gasification syngas at relevant operating conditions. Effects of tar model (toluene) presence on catalyst deactivation and coke formation were studied. The fresh samples were characterized by ICP-MS, XRD, TPR, N2-physisorption and H2-chemisorption. The coke formation was studied by TGA-TPO, Raman spectroscopy and SEM/EDS. Efficient formation of Ni-Co alloy particles was confirmed by XRD and SEM/EDS. The Ni-Co/Mg(Al)O system shows high steam reforming activity with complete model tar removal. Simultaneous high-temperature H2/CO ratio adjustment by effective equilibration of the WGS reaction was demonstrated. Initial Co addition was proposed to reduce the formation of strongly deactivating encapsulating coke species by efficient removal of coke precursors at the expense of enhanced carbon filament formation. Lower Ni-Co ratios were shown to effectively reduce deactivation and coke formation at the expense of the higher initial activity of the high Ni-Co ratio system. Restructuring of extensive carbon filament clusters was proposed as an additional route to encapsulating coke species with potential long-term deactivation effects. The Ni-Co ratio was shown to affect the carbon filament formation rates and filament diameter distributions. The effects on coke formation were proposed to depend on complex interactions of metal particle size and Ni-Co alloy ratio. The results contribute to the understanding of the highly attractive resistance towards deactivation by coke formation of Ni-Co catalyst systems.