An automatic absorptiometric method is presented for the determination of silicon in iron ores, sinters, slags, iron and steel. The last two are dissolved in dilute sulphuric acid; the others are fused with sodium peroxide, then dissolved in hydrochloric acid. After suitable additions and dilution, the resulting solutions are treated identically in a Technicon AutoAnalyzer. The silicomolybdate formed is reduced to molybdenum blue with iron(II). Fluoride is added to provide a redox buffering system. The novelty of the method lies in selecting the sample sizes, conditions of fusion and/or dissolution and dilutions so that this universality is attained. Thus a single programme for the analyzer serves to determine, in any sequence, silicon in iron ores or sinters (1-6%), slags (3-10%), iron (0.4-2.5%) or steel (0.005-2%), the only change being in the tables used to translate transmittanee into per cent silicon. Both the precision and accuracy are satisfactory.