This study aimed to survey the content of Maillard reaction products in different UHT-treated milk products and to assess how formation of these products and lysine blockage is affected by the composition. For this purpose, different commercial UHT milks including milk (whole, semi-skimmed and skimmed), lactose-hydrolyzed, protein-fortified, lactose-hydrolyzed protein fortified and follow-on infant milks were analyzed. Among the Maillard reaction products, dicarbonyl compounds, 5-hydroxymethylfurfural, furosine, N-epsilon-carboxymethyllysine and N-epsilon-carboxyethyllysine were monitored. The results showed that fortification of UHT milks with protein and carbohydrates as well as hydrolysis of lactose promoted the Maillard reaction. Among the dicarbonyl compounds, 3-deoxyglucosone formation, which was the dominant dicarbonyl compound in milks, varied between 3.12-12.67 mg/L, 13.45-21.98 mg/L and 4.59-40.38 mg/L in lactose hydrolyzed, lactose-hydrolyzed protein fortified and follow-on infant milks whereas it was 0.22-0.40 mg/L in milks, respectively. Similarly, 5-hydroxymethylfurfural could not be detected in milks, whereas mean 5-hydroxymethylfurfural concentration was found to be 56.3 mg/L and 31.5 mg/L in protein-fortified milks and lactose-hydrolyzed protein-fortified milks, respectively. Accordingly, % blocked lysine, furosine, N-epsilon-carboxymethyllysine and N-epsilon-carboxyethyllysine content of different UHT milks were found to be significantly higher than milks (p < 0.05). This is the first study reporting the Maillard reaction products in protein-fortified and infant milks.