The pH titration of the spectroscopic probe molecule 4-octadecyl oxy-1-naphthoic acid (ONA) cast in Langmuir-Blodgett films with the neutral diluent molecules polyvinyl octadecyl ether (PVOE) and eicosan-1-ol, onto hydrophobic silica, has been examined via the technique of attenuated total internal reflection spectroscopy and streaming potential measurements. The ONA molecules exhibited pronounced pH-dependent aggregation when cast onto hydrophobic silica plates. Neutralization (titration from high to low pH) of charged ONA films was in agreement with the behavior predicted by the Gouy-Chapman equation. Reionization (titration from low to high pH) of the ONA molecules proceeded much more slowly than neutralization and the rate was also slower in the polymeric PVOE diluent. Marked hysteresis was evident between the neutralization and reionization curves, this being more pronounced at lower ionic strength and less pronounced in the polymeric PVOE diluent. The reionization results could be modeled using ONA dimers to represent the aggregation state of the probe molecules in the film at low pH.