Tetracyclines constitute one of the most important antibiotic families, ranking second in production and usage worldwide. Tetracyclines have been detected in many environmental compartments. There is very little information available on the interaction of tetracyclines with dissolved organic matter, which may play an important role on its environmental mobility and transformation. In this study, equilibrium dialysis method was used to investigate the solution chemistry (pH; sorbate-to-sorbent ratio; ionic strength) effects on the interaction of tetracycline with Elliott Soil humic acid (ESHA). Our results showed that sorption equilibrium with ESHA was attained in 24 h. A similar equilibration period was required for tetracycline even in the absence of humic acid, which suggests that the rate-limiting step for the sorption process was antibiotic diffusion through the dialysis membrane. The extent of sorption was strongly pH-dependent and exhibited a maximum at around pH 4. The macroscopic data were successfully described by a discrete log K model using the FITEQL 4.0 chemical equilibrium program. Tetracycine sorption could be occuring due to the complex formation between its cationic species and the deprotonated ligands in ESHA. The results obtained would complement well with those from our earlier study on tetracycline sorption to iron and aluminum hydrous oxides.