This is an investigation on the effects of pyrolysis atmosphere on the conversion of biomass/coal blends to chars and volatile matter, and on the release of sulfur and nitrogen to the gas phase. Pulverized corn straw and lignite coal were co-pyrolyzed either in an inert carrier gas (N2) or in oxidative gases consisting of 3–7% O2 and 13–17% CO2 in a balance of 80% N2. The process occurred at temperatures in the range of 300–900 °C, under 1 atm, and lasted for 20 min. The results showed that the solid-phase conversion of coal/biomass blends ranged from 30 to 60%, increasing with increasing temperature. Increasing oxygen concentration under oxidative atmospheres promoted the solid-phase conversion at 500–900 °C, more than under the inert atmosphere, but had no discernible effect at lower temperatures (300–400 °C). Under the inert atmosphere, the co-pyrolysis of coal and biomass exhibited a synergistic effect, which promoted the transformation of solid feedstock into the gas phase or tar. To the contrary, under the oxidative atmospheres, the co-pyrolysis inhibited the evolution of volatile matter. Sulfur compounds, COS and SO2, were mainly released at temperatures in the range of 300–700 °C. Higher amounts of SO2 were released under the oxidative atmosphere, and the co-pyrolysis of coal and biomass promoted the release of COS. As for the transformation and release of nitrogen, NH3 and HCN were mainly released in the temperature range of 600–900 °C and more HCN was converted into NH3 under inert atmospheres. The effects of the oxygen content in the atmosphere on conventional gas species (CO, CH4, C2H6, and HCl) released from coal/biomass blends were also recorded.