Sodium pyrosulphite, also known as sodium metabisulphite, is useful as a convenient solid source of sulphur dioxide and, in particular, has a wide market possibility as a preservative for silage. It can be prepared by treating any basic sodium compound with sulphur dioxide. The particular case examined in the present report is based on starting with the crude sodium bicarbonate intermediate in a soda-ash manufacturing operation. Two possible manufacturing routes are explored: preparation in solution via a recycling magma of concentrated sodium pyrosulphite solution, and preparation in a semi-dry process by direct reaction with the moist bicarbonate cake.It is concluded that the product could easily be produced at the current U. S. price of 6.6 cents/lb FOB. The primary market for local production would be as a silage preservative and since this would be a new use in western Canada, some further market studies are indicated.