The interfacial area achem in a gas-liquid contactor as determined by the chemical method deviates from the true geometrical interfacial area ageo, because the overall conversion of the gas phase reactant represents an incorrect average if bubble sizes and residence times are not uniform. The deviations of achem fromageo become larger the broader the distribution τb/db and the higher the overall conversion ΩA of the reactant in the gas phase. Model calculations, which take into account both the effect of gas phase backmixing as well as the effect of bubble coalescence on the deviation of achem from ageo, are performed for a mechanically agitated gas—liquid reactor and a bubble column at practical micro- and macromixing conditions. For a gas-liquid model reaction, which is first-order in the gas phase reactant, it is found that: (1) for a mechanically agitated reactor the error in achem will always be smaller than 10% if ΩA is lower than 0.99, and (2) for a bubble column the error in achem will be smaller than 20% for most practical applications if ΩA is lower than 0.99. Gas-liquid model reaction systems with absorption of CO2 in alkanolamine solutions are recommended for the determination of interfacial area in gas—liquid contactors.