Experiments showed that the intraoral iodide permeability (Ip) method can be used in a reproducible and sensitive manner with solid foods. Ingestion of 5-gram portions of cookies made with defined concentrations of sucrose or fat led to an increased Ip (due to demineralization) of Streptococcus mutans-covered bovine enamel blocks in vivo. Demineralization increased with time to a maximum of 45 min, and the pH of the plaque dropped accordingly. Continued exposure in the mouth beyond 45 min led to an elevation of the pH and a decrease in delta Ip consistent with remineralization of the enamel. Control blocks worn without ingestion of cookies exhibited negative delta Ip values. Demineralization increased with increasing sucrose content of the cookies and reached a plateau when cookies containing 1.08 g sucrose per morsel were administered. Cookies prepared without added sucrose gave a high delta Ip. High fat content raised the delta Ip when sucrose was low. These findings are consistent with clinical and other observations, and emphasize the complex relation between foods and enamel demineralization.