This paper reports the in situ study of conformations of flexible synthetic polymer molecules of protonated poly(2-vinylpyridine) at the solid-liquid interface at different pH of aqueous solutions. The direct conformation study of such thin ( approximately 0.4 nm) single molecules under liquid is performed for the first time. The highly protonated poly(2-vinylpyridine) chains possess a conformation of 2D equilibrated random coil, while at low degree of protonation, molecules are in the conformation of a strongly compressed 3D coil. Molecules are immobile during the few hours of the AFM experiment.