Phase equilibrium behaviour of antibiotics is important in drug design and for optimization of the recovery process in manufacturing. Aqueous/organic partitioning behaviour of a clinically important antibiotic, tetracycline is measured. The organic phase includes pure solvent n-hexane, chloroform, diethyl ether and ethyl acetate. Measurements are also made for mixed organic phase composed of n-hexane + ethyl acetate. Organic-phase partitioning increases with the hydrogen bonding tendency of the solvent in the order n-hexane < chloroform < diethyl ether < ethyl acetate. An activity coefficient model that includes hydrogen bonding, UNIQUAC-HB [R.B. Gupta, R. Kumar, G.V. Betageri, Phase behaviour of mixtures containing antibiotics, chloramphenicol partitioning, Ind. Eng. Chem. Res., 36 (1997) 3954-3959.], is used to model the partitioning behaviour between aqueous and organic phases. Single solvent-organic-phase data are correlated using the model to obtain the free-energy-of-hydrogen-bonding parameters for hydrogen bonding between antibiotic and solvent. Predictions made for mixed-solvent-organic-phase systems agree well with the experimental data without using any adjustable parameter.