Fatty acids containing a system of three conjugated double bonds in their structure (TCFA, i.e. trienoic conjugated fatty acids) are products of lipid oxidation and appear harmful to infant health; as such, they can be used as determinants to determine a degree of acid oxidation and its progress in the infant formula IF and FF (follow-on formula). The TCFA content was determined in a free and bonded acid contained in IF and FF, and expressed numerically as a ‘K’ parameter [%]. The ‘K’ parameter is a ratio between the absorbance of the system of three conjugated double bonds in fatty acids (the analytical wave length is 256 nm) and the absorbance characteristic for carbonyl groups of triacylglycerols (218 nm). A reference sample was acid contained in the mature human milk. The maximum ‘K’ parameter value of human milk was 0.60 ± 0.03% (N=9) whereas its range in the case of IF and FF studied was between 4.49 ± 0.27% (minimum) and 12.40 + 0.73% (maximum) and depended on the manufacturer of IF and FF (there were 5 manufacturers who supplied 17 products in total). Discrepancies in the progress of lipid oxidation degree are to be attributed to the quality and quantity of plant oils added to the formulas, and to the manufacturing technology applied to produce IF and FF. Because of the harmfulness of lipid oxidation products, especially to infants and little children, it was suggested that the ‘K’ parameter of fat as contained in IF and FF should be limited to 6% at the most. From among all 17 studied samples, 22% met this requirement.