Patients who believe they have oral malodour often have a dry mouth condition instead. Here we have examined its relation to oral malodour, real or perceived. A direct relationship between the thickness of the film of residual saliva on mucosal surfaces throughout the mouth and perception of a dry mouth was observed. On the hard palate, the thickness of this film proved to be diagnostic for a dry mouth and corresponded to lower resting saliva flow and pH levels (P< 0.001). Intra-muscular administration of the anti-sialogogue, Robinul, accurately produced the dry mouth condition. Using a sulphide monitor, loss of volatile sulphur compounds into mouth air progressively occurred as the mouth became drier. Mouth pH and Eh on the dorsum of the tongue correspondingly fell. Mouth breathing led to tongue and palate moisture loss thus enabling escape of malodour volatiles into mouth air. Measurement of oral dryness should make it possible to differentiate genuine malodour from dry mouth related pseudo-malodour subjects, and in turn, the latter from patients that are halitophobic. This should facilitate identification of such patients and avoid error in their clinical management.