The azide ion is a strong physical quencher of singlet molecular oxygen (¹O₂) and is frequently employed to show involvement of ¹O₂ in oxidation processes. Rate constants (k_q) for the quenching of ¹O₂ by azide are routinely used as standards to calculate k_q values for quenching by other substrates. We have measured k_q for azide in solvent mixtures containing deuterium oxide (D₂O), acetonitrile (MeCN), 1,4-dioxane, ethanol (EtOH), propylene carbonate (PC), or ethylene carbonate (EC), mixtures commonly used for many experimental studies. The rate constants were calculated directly from ¹O₂ phosphorescence lifetimes observed after laser pulse excitation of rose bengal (RB), used to generate ¹O₂. In aqueous mixtures with MeCN and carbonates, the rate constant increased nonlinearly with increasing volume of organic solvent in the mixtures. k_q was 4.78 × 10⁸ M⁻¹ s⁻¹ in D₂O and increased to 26.7 × 10⁸ and 27.7 × 10⁸ M⁻¹ s⁻¹ in 96% MeCN and 97.7% EC/PC, respectively. However, in EtOH/D₂O mixtures, k_q decreased with increasing alcohol concentration. This shows that a higher solvent polarity increases the quenching efficiency, which is unexpectedly decreased by the proticity of aqueous and alcohol solvent mixtures. The rate constant values increased with increasing temperature, yielding a quenching activation energy of 11.3 kJ mol⁻¹ in D₂O. Our results show that rate constants in most solvent mixtures cannot be derived reliably from k_q values measured in pure solvents by using a simple additivity rule. We have measured the rate constants with high accuracy, and they may serve as a reliable reference to calculate unknown k_q values.