The molecular structures of the interfaces between a solid poly(4-vinyl pyridine) (P4VP) surface and poly(acrylic acid) (PAA) as well as hydrochloric acid (HCl) solutions were probed using sum frequency generation (SFG) vibrational spectroscopy in situ in real time. Spectroscopic results clearly reveal that the PAA molecules are adsorbed onto the P4VP surface via hydrogen bonding at the P4VP/PAA solution interface while the P4VP surface is protonated at the P4VP/HCl solution interface. Consequently, the water molecules near the interfaces are strongly perturbed by these two interactions, exhibiting different orderings at the two interfaces. This work clearly demonstrates the power of studying the interfacial molecular-level structures via nonlinear vibrational spectroscopy when molecular adsorption happens at the solid-liquid interface and paves a way for our future study on tracing the adsorption dynamics of polymer chains onto solid surfaces. (C) 2015 Wiley Periodicals, Inc.