The sample is ashed in a nickel crucible over an open ﬂame with calcium hydroxide as thefixative; ﬂuoride is separated by micro-diffusion and measured with a ﬂuoride-selective electrode.The calcium hydroxide fixative permits a temperature of 800—850°C during the final ashing, sothat insoluble silicates are decomposed and ﬂuoride is converted to an acid-soluble form. Otheradvantages over furnace ashing at 600°C are less contamination and more rapid ashing (20 mincompared to 6 h). The interference of aluminium and silica on microdiffusion was studied: per-chloric acid was found to be preferable to sulphuric acid as medium. A new way for preparingcalcium hydroxide suspension, low in ﬂuoride, is described. The values found by the proposedmethod generally compare well with those obtained by oxygen ﬂask combustion. Direct diffusion(i.e. without ashing) gave much lower values than the ashing methods, except for tea leaves.