The objective of the investigation was to determine the relationship between the content of acids, in particular of trans isomers (TFA) and chemical changes occurring in fats during storage of puff-pastries manufactured with those fats added. The range of the investigation involved two assessments: the assessment of chemical properties of seven bakery vegetable fats which differed, specifically, in the content of trans isomers (from 4,1 to 54,2%), and the assessment of hydrolytic and oxidative changes occurring in fats contained in puffpastries during a 49-day storage of these products. The fatty acid content in fats was determined using a gas chromatography. The fat was extracted using a Kates method, and an acid value and peroxide value in extracted fats were determined. On the basis of the investigation accomplished, no significant relationship was stated between the content of trans isomers of fatty acids and the hydrolytic and oxidative changes in fats occurring during storage of puff pastries. However, it was noted that the rate of increase in the quantity of free fatty acids in fats containing the highest level of trans isomers (36,54%) was, relatively, the slowest.