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It has been appreciated for some time that low oxygen activity is required for effective steel desulphurization, and recently this has been recognized for hot metal desulphurization. In the present experiments, calcium carbide was injected into 70 kg melts of iron during which desulphurization and deoxidation were studied by continuously measuring oxygen activity and frequently sampling for sulphur content. In some experiments, carbon dioxide was used either as a carrier gas or as generated from calcium carbide/limestone mixtures. The experiments confirmed that the oxygen activity in hot metal was controlled by the silicon-silica equilibrium, except when aluminum was added. It was found that the rate of desulphurization was associated with the oxygen activity. The desulphurization rate increased as oxygen activities decreased in order from carbon dioxide-containing injections into iron-carbon-silicon melts, to calcium carbide/nitrogen injections into iron-carbon-silicon melts, to finally calcium carbide/nitrogen injected into iron-carbon-aluminum melts. The dependence of the incubation period with the melt oxygen activity was recognized. The incubation times coincide with periods of deoxidation before desulphurization proceeds. In order to interpret the experimental results, a kinetic model was developed that takes simultaneous desulphurization and deoxidation into account. This model demonstrates clearly that the rate of desulphurization is controlled by the melt oxygen activity and the powder feed rate. Thus, low oxygen is required for effective hot metal desulphurization. In a log h$\sb{\rm o}$-log h$\sb{\rm s}$ diagram, the refining trajectories determined from the kinetic model were found to be consistent with those obtained from the experiments. From the model calculations, it was found that the commercial calcium carbide which normally contains 30 per cent of lime can be employed for the melts pre-deoxidized with silicon or aluminum, without causing signif, Doctor of Philosophy (PhD)