Saliva coating all oral surfaces has a buffering capacity that neutralizes bacterial and cariogenic acids. The aim of our study was to determine the surface pH in different regions of the oral cavity in healthy volunteers and in patients with diseases affecting the oral mucosa. Oral pH was measured with a flat glass electrode on the anterior third of the ventral surface of the tongue, middle hard palate, buccal mucosa and inner lips in 32 healthy volunteers, 12 patients with Behçet's disease, 23 patients with oral lichen planus, and 11 patients with burning mouth syndrome. The present study showed that there was an uneven distribution of oral surface pH. The palate had a higher pH than most other sites in all groups, and in patients with lichen planus, the palate pH was higher than that in healthy controls. Those with dentures had lower pH values in the hard palate than dentate patients. The relatively high pH in the palate region in all patient groups as well as healthy volunteers needs to be further studied to clarify its mechanisms and clinical relevance.