Despite the potential for biomass pyrolysis to produce renewable fuels, the governing chemical reactions are largely unknown due to the complexity of biopolymers, such as cellulose. In this work, we use isothermal pyrolysis experiments to reveal the chain length (or end-group) effect that controls the distribution of pyrolysis products from linear β-1,4-glucan polymers (e.g., cellulose). Finally, we show that a simplified end-group/interior monomer model is largely incapable of predicting product yields from cellodextrin pyrolysis.