The aim was to study the behavior of chlorogenic and caffeic acids in solutions of electrolytes for simulated digestion in the mouth, stomach and small intestine, at different pH values. Chlorogenic, neochlorogenic, cryptochlorogenic, and caffeic acids were prepared in electrolyte solutions, and analyzed using the RP-HPLC (Reversed-Phase High-Performance Liquid Chromatography) method at the beginning and after 4 h of incubation at 37°C. Extraction of polyphenolic compounds from apples and simulated digestion of apples in vitro were also carried out, in order to see the behavior of chlorogenic acid in the real sample. There were no visible changes in acids immediately after dissolution in model electrolyte solutions (t=0). After 4 h of incubation in the electrolyte solution for simulated digestion in the mouth and small intestine (pH 7), the isomerization of all chlorogenic acids was visible, while caffeic acid did not show the presence of other acids, however it almost completely disappeared (electrolytes of the small intestine). The percentage distribution after 4 h at pH 7 (mouth, small intestine) was as follows: after incubation of chlorogenic acid (53 - 78% chlorogenic, 7 - 11% neochlorogenic, 14 - 15% cryptochlorogenic acid), after incubation of neochlorogenic (33 to 44% neochlorogenic, 13% chlorogenic, 24 - 31% cryptochlorogenic acid), after incubation of cryptochlorogenic (16 to 23% cryptochlorogenic, 20 - 30% neochlorogenic, and 13 - 17% chlorogenic acid). Only 1-30% of caffeic acid remained in its original form. In the simulated gastric electrolyte solution, a higher stability of the tested acids can be suggested. Chlorogenic and p-coumaroylquinic acid were identified in the apple before digestion. In addition to those acids, additional isomers were identified (after digestion in the stomach cryptochlorogenic acid, after digestion in the small intestine neochlorogenic, cryptochlorogenic, and unidentified phenolic acid) which shows that isomerization of chlorogenic acid also occurs inthe real sample. The results might be important for the bioeffects of chlorogenic and caffeic acids in the human body.