| Summary: | The investigation of the adsorption and diffusion properties of polylactic acid (<i>PLA</i>) is very important, because during the application of the polymer, interactions can occur between the polymer and its environment that can affect its properties and application. The aim of our work was to investigate a method for determining the solvent sorption capacity of <i>PLA</i> and to investigate how different additives, such as essential oils, affect the solvent sorption properties of the polymer. Experiments were carried out to explore the correlation between the solvent uptake of two different types of <i>PLA</i> granules and the solubility parameter of the selected 5 essential oils (<i>Melissa officinalis</i>, <i>Mentha piperita</i>, <i>Foeniculum vulgare</i>, <i>Majorana hortensi</i>, <i>Thymus vulgaris</i>) for 3 solvents. It was observed, that application of essential oils was changed the solvent uptake of the granules differently. While one granule solvent uptake decreased on average by 2–3 wt.%, the other increased by a similar amount. The difference of sorption capacity of pure and essential oil containing solvent were between 20–190%. The specific essential oil uptake was highest in solutions with a concentration of 2.00 mg/mL, about 2.00 mg <i>EO</i>/g <i>PLA</i>. In alcoholic solutions we observed a relation between the solvent uptake of <i>PLA</i> and the solubility parameter of the relevant essential oil.
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