Up to 2 mg/l of vitamin B₁₂ were produced batch-wise using Propionibacterium freudenreichii on an ammonia based spent sulfite liquor medium in 250 ml Erlenmeyer flasks and a 7-litre benchtop fermentor. The up to 6 gm/l of bacteria utilized the hexoses from the SSL, thus reducing the BOD by 50-80% and producing, as by-products, 3 gm/1 of acetic acid and 7 gm/1 of propionic acid. Pre-treatment of the liquor required stripping of SO₂ to below 200 ppm and adjustment of the pH to between 6.5 and 7.5; precipitation of 1igno-su1fonate was not necessary. It was necessary to add excessive amounts (up to 75 gm/1 dry) of yeast extract or other supplementary nutrient to achieve these vitamin B₁₂ yields. Solutions to this nutrient problem are suggested. The Monod fermentation model whose parameters were estimated using non-linear least squares techniques with dry bacterial cell concentration as the dependent variable, approximate the batch data well. Other less complicated models were not as satisfactory. Estimating the model parameters using linear techniques was most unsatisfactory indeed. Optimum hold-up times for a single stage fermentor for Propionibacteria production, predicted from the Monod model whose parameters were estimated from batch data, were up to 125 hours. These hold-up time extrapolations are subject to large errors. Recommendations for further work on the vitamin B₁₂ process, the extension of the batch modeling work to continuous fermentation and further work on other more suitable microbiological products made from SSL are entertained. / Applied Science, Faculty of / Chemical and Biological Engineering, Department of / Graduate