Lactic acid (2-hydroxypropanoic acid) is a significant platform chemical for the biorenewable economy. Concentrated aqueous solutions of lactic acid (> 30 wt.%) contain a distribution of oligomers that arise via intermolecular esterification. As a result, the titratable acidity changes non-linearly with acid concentration. In this work, the oligomer distribution of lactic acid is characterized using GC, GUMS, and HPLC to extend existing literature data, and titratable acidity is measured via titration with NaOH. A thermodynamic model with a single parameter is proposed that accurately represents oligomer distribution and titratable acidity over the full range of lactic acid concentrations. (c) 2005 Elsevier B.V. All rights reserved.