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 m(3)/m(2)h 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: us well as to summarize how the effective interfacial area can be calculated in order to motivate other researchers to publish interfacial area instead of k(OG)(.)a, when using Co-2-NaOH as test system.