The aim of this article was to investigate the effects of methanehydroxydiphos-phonate (MHDP) and fluoride (F) separately and combined, on the demineralization process in enamel and in dentine under constant composition conditions. The demineralization was carried out in solutions containing 3 mM calcium, 3 mM phosphate, pH = 5.0, and either 6 μM MHDP, 12 μM F or 6 μM MHDP and f 2 μMF. After demineralization periods (t) between 24 and 140 h, the lesion depth 1 and the mineral loss ΔZ were measured by transverse microradiography of the enamel and dentine speciments. The amount of mineral lost during demineralization averaged over the lesion depth, R = ΔZ/1, was calculated. R ≈ 17 vol% for both enamel and dentine, both ΔZ and 1 being greater for dentine than for enamel by approximately the same factor. The results show that under the same conditions dentine is attacked much more strongly than enamel. The lesion progress with time is quite different for the two tissues: in enamel the lesion depth progresses als l3 = αt + β, in dentine as l2 = α’t + β‘. The slopes α and α’ decrease in the order MHDP > F > MHDP + F. In enamel the combined inhibitor effect is only slightly larger than the effect of F alone; in dentine the inhibiting effects of F and MHDP are approximately additive. The inhibitor interactions can be described by the Langmuir adsorption theory. For the adsorption of F onto enamel the adsorption constant Kkin was calculated to be 5.2 × 104 liters/mol, about an order of magnitude smaller than the corresponding value for hydroxyapatite. For adsorption of MHDP onto enamel, Kkin was found to be 1.0 × 104 liters/mol, which is in the same range as the corresponding value for hydroxyapatite.