The effective interfacial area of the Nutter Ring NR#1, NR#1.75 and NR#2.5 was measured by absorbing CO2 from air with NaOH. Liquid holdup was measured using air-water as test system. The liquid load was varied between 20 and 180 m3/m2h and the influence of the pressure drop was investigated. Correlations are derived from the experimental data which represent the measured data within an accuracy of 10 %. The aim of this work is to present the test results as well as to summarize how the effective interfacial area can be calculated in order to motivate other researchers to publish interfacial area instead of kOG · a, when using CO2-NaOH as test system.