Plums are very popular fruits in Poland. They are a rich source of phenolic compounds and stand out for their high antioxidant capacity. Seasonality of supply of plums determines the processing of plums. This, in turn, impacts qualitative and quantitative changes in the compounds contained in plums. The process of drying plums causes the content of phenolic compounds to decrease and, simultaneously, the antioxidant capacity of those fruits to increase. Products of the Maillard reaction and/or of the caramelization that are formed during the thermal processing account for those changes. Those products show both the positive and the negative effects on human health. When having the knowledge of changes in the chemical composition of plums that occur during processing operations of plums, it would be possible to develop appropriate fruit drying methods. Another good reason for doing so is that the plum fruits cultivated in Poland are characterized by a high content of extract and dry matter. Unfortunately, they are rarely used by the local industry. The identification of changes induced by processing operations could help dried plum products produced become more competitive on the Polish market.