Luminosity of pool fires from methanol/additive mixtures is investigated theoretically and experimentally. Phase diagrams of solutions are used to explain the transient behavior of flame luminosity of binary fuel mixtures. A model, based on solution theory, is developed to quantify the flame luminosity of methanol/additive binary fuel mixtures from the luminosities of the individual components and an interaction coefficient. The theoretical results agree well with the experimental data. The luminosity of methanol additive mixtures is found to be a function of the mixture's carbon/hydrogen (C/H) ratio. Flame luminosity increases with increasing mixture C/H ratio to a maximum (occurring around C/H = 0.35), and then decreases slightly beyond that point. Additives that form positive azeotropes with methanol and that have high C/H ratio are found to be the best candidates for enhancing the luminosity of methanol flames.