The objective of the research study was to determine the effect of fat products with varying degrees of unsaturation on the stability of thiamine hydrochloride. Among the selected fat products, there were: refined rapeseed oil, refined sunflower oil, linseed oil, extra virgin olive oil, and pork lard. An amount of 0.1 mg/100 g of thiamine hydrochloride solution was deposited onto a cellulose and, then, the cellulose was mixed with selected fat products that were added in the amount of 50 %. Samples were stored at a temperature of 30 ºC. In the periodically collected samples, the thiamine content was determined using a thiochromium method as were oxidative changes based on the contents of primary (peroxide value) and secondary (anisidine value) oxidation products. In the fat products, the fatty acid composition was determined. Unsaturated fatty acids: C18:1, C18:2, and C18:3 constituted ca. 90% of total fatty acids in the oils and 60% in the lard. It was found that the type of fat product had a significant impact on the stability of thiamine hydrochloride. The presence of fats showing a higher degree of unsaturation was correlated with the higher rate of oxidative changes during the incubation and, at the same time, with the higher losses of the thiamine hydrochloride. The highest disintegration rate of thiamine was found in the samples with linseed oil and the lowest in the samples with lard. Based on the time period necessary for thiamine losses of 50% (T_1/2) to be incurred, the results were arranged as follows: linseed oil (T_1/2 = 28 days) > sunflower oil > rapeseed oil (T_1/2 = 79 days) = olive oil > lard (T_1/2 = 93 days). The losses of thiamine hydrochloride during the storage correlated with the production of fat oxidation products, in particular with the content of the secondary products of that process.