The possibility of absorption determination of oxidised and unoxidised fatty acids with conjugated double bonds and of chain degradation products, such as aldehydes or ketones was investigated. The samples of rapeseed oils with different thermooxidative transformations after longterm potatoes frying were studied. The study showed that for fats with deep thermooxidative changes, measurements of UV absorbance at 233 and 268 nm can not be used directly for determination of content of fatty acids with conjugated double bonds. Their quantitative determination is possible only after removing of carbon chain degradation products and after reduction of hydroperoxides and ketoacids with conjugated double bonds followed by dehydration of hydroxy acids. During the last reaction, transformation of hydroxy acids to acids containing one more conjugated bond takes place.