Without oxygen, there would be no combustion. Yet the crucial role of oxygen in energy systems has been largely underrepresented. The oxygen-dependence of fuel caloricity is used to derive energy-based metrics that challenge the prime facie objective of maximising biofuel mass yieldsdan objective that currently dominates the biofuel industry. Application of two energy metrics, namely the change in energy quality (Delta E-Q) and the energy yield (Delta E-eta), demonstrates that any improvement in energy quality of combustibles must accompany increases in combusted oxygen (m(O2)), as Delta E-Q = m(O2)vertical bar product/m(O2)vertical bar feed - 1, and that Delta E-eta m(O2) is a function mass yield, mu, and m(O2): Delta E-eta = eta(m(O2)vertical bar product/m(O2)vertical bar feed). Literature data produced mostly positive Delta E-Q values: biocrude achieved the highest (126:3 %), followed by bioethanol (107:7 %), and catalytic pyrolysis (78:3 %). Most data produced similar changes in energy yield per mass yield, Delta E-eta/Delta m(eta) ranging from 0.7 for biodiesel to 1.6 for bioethanol. Carbon yields and overall mass yields are demonstrated to be poor metrics for biofuel synthesis, and could undermine biofuel quality, hampering progress for conversion technologies aimed at producing biofuels. Biodiesel mass yields in particular were found to be inflated by as much as 73 %. As such, Delta E-Q and Delta E-eta are useful for cross-field comparison of biofuels. (c) 2020 Elsevier Ltd. All rights reserved.