The state-of-the-art of the gas-liquid mass transfer characteristics in trickle-bed reactors was summarized and its quantification methods were reevaluated based on a wide-ranging data base of some 3200 measurements. A set of three unified whole-flow-regime dimensionless correlations for volumetric liquid- and gas-side mass transfer coefficients, and gas-liquid interfacial area, each of which spanned four-order-of-magnitude intervals, were derived. The correlations involved combination of artificial neural networks and dimensional analysis. The dimensionless interfacial area, Sit, and Sit, were expressed as a function of the most pertinent dimensionless groups: Re-L, Re-G, We(L), We(G), Sc-L, Sc-G, St(L), X-G, Mo-L, Fr-L, Eo(M), S-b.