Lesion formation on root surfaces of human posterior teeth was studied in acetate/lactate buffers with a background electrolyte composition based on plaque fluid analyses. Lesion depth after 28 days at 37°C was measured in relation to: the presence or absence of cementum; the concentration of undissociated buffer; the presence or absence of magnesium ions at plaque fluid concentration. Each factor was evaluated at several values of –log(ion activity product for hydroxyapatite): pIHA. Solutions were formulated to minimize variation in pH, which varied by ≤0.03 for a given comparison (individual pIHA) and by 0.42–0.82 over the range of pIHA within experiments. Lesions on surfaces from which cementum had been ground were significantly deeper than on intact surfaces, but this is considered to be due to subsurface mechanical damage and not to a solubility difference. Neither the concentration of undissociated buffer nor the presence of magnesium ions significantly affected lesion depth. Lesion depth was strongly influenced by the correlated variations in pIHA and pH. At pIHA 54 and 55, only extremely shallow lesions formed. From pIHA 56, lesion depth increased with increasing pIHA. The results confirm that the solubility of the mineral of root tissues is higher than that of hydroxyapatite, but indicate that it is probably lower than suggested by Hoppenbrouwers et al. [Arch Oral Biol 1987;32:319–322]. For calcium concentrations of 3–12 mM, the critical pH for root tissue mineral was calculated as 5.22–5.66 assuming solubility equivalent to pIHA 54 and 5.08–5.51 assuming pIHA 55.