The structures and properties of N-alkyl-substituted polyacrylamides (PAA) were studied by FT-IR spectroscopy. For N-alkyl-substituted PAA with lower critical solution temperature (LCST), a specific H-bonded cyclic structure involving alkyl amide units of a polymer and hydroxyl functions of water was observed and found responsible for polymer solubility. The mechanism for the appearance of a LCST in aqueous solution of N-alkyl-substituted PAA is discussed and factors of position of LCST are examined.