The chemical modifications occurring to the multitude of compounds formed in oils during frying after ingestion and prior to absorption are still unknown. The objective of this work was to explore the depolymerization and oxidation events which may occur under simulated gastric conditions and obtain quantitative data of the compounds formed. Samples of used frying sunflower oil with increasing alteration degree were selected for in vitro digestion. The methodology applied to determine changes in triacylglycerols (TAG), oxidized TAG monomers (oxTAGM), TAG dimers (TAGD) and higher oligomers (TAGO) consisted of a combination of adsorption and size exclusion chromatographies while changes in epoxy, hydroxy and keto fatty acyls were evaluated after oil transesterification by combination of adsorption and gas–liquid chromatographies. Among the results obtained, the large extent of depolymerization after digestion at pH 1.2, reaching levels as high as 70%, stood out. The release of unoxidized TAG from polymeric molecules was reflected in their significant increase after digestion. Hydroxy fatty acid methyl esters significantly increased in all samples after digestion. These results demonstrated that relevant structural modifications may occur to the compounds found in frying oils during gastric digestion. Further investigation is crucial to assess the potential health implications of the compounds formed.