The project has demonstrated the application of liquid chromatography coupled to mass spectrometry (LC-MS) for the characterisation of oligolactic acids (OLAs), employed as pharmaceutical excipients in metered dose inhalers. OLAs proved to be difficult to characterise because of their complexity, which was ascribed. to the presence of repeated structural monomeric units and a nonrepeating moiety (the head group). Furthermore, during the course of method development the potential presence of degradation products and impurities had to be considered for quality control purposes. Various LC-MS methods were developed to target both oligomeric distribution and head group functionalities of OLAs. Liquid chromatography at critical conditions (LCCC), aimed at addressing the head group distribution of OLAs, led to the separation of the cyclic impurities from the parent linear molecules. However, to successfully achieve a complete characterisation of OLAs, a second separation targeting the oligomeric distribution was investigated. Hydrophobic and polar interactions and possible solvation effects, which regulate RP-HPLC separation mechanisms, proved to be able to offer the selectivity necessary to resolve OLAs in terms of their size and their head groups, leading to the simultaneous separation between the linear molecules and their cyclic impurities and the determination of oligomeric distribution.