In studies of the pH response of dental plaque in situ to rinses with conventional sugars, glucose and maltose give similar falls in pH, and fructose only slightly smaller effects compared with sucrose. Lactose and galactose are less acidogenic, while the pH response to glucose syrups varies according to their composition. Of the sugar alcohols, sorbitol, mannitol, maltitol and lactitol are all slowly fermented to acid by oral bacterial, and xylitol is virtually non-fermentable. Acid formation in plaque by fermentable sugar alcohols can be too slow to overcome the buffering power of plaque and saliva and plaque pH can rise following exposure to these compounds. Lycasin, a synthetic sweetener containing sorbitol, maltitol and some higher sugar alcohols gives effects similar to its major constituents. Palatinit, L-sorbose and trichlorogalactosucrose are of low acidogenicity, but coupling sugar is more fermentable and can give rise to a substantial pH response, albeit less than sucrose. Non-nutritive intense sweeteners may affect plaque pH by their sialogogue effects.