The decomposition of a mixture of organic compounds was studied by measuring the evolution of carbon dioxide, and changes in the concentration of ammonium, sulphate, and phosphate. In one experiment the nitrogen supply was varied by varying the proportion of glycine in the mixture of organic compounds; in another the sulphur supply was varied by varying the proportion of cysteine; and in a third the phosphorus supply was varied by varying the proportion of sodium β-glycerophosphate.Mineralization of an element depended on the concentration of that element in the organic mixture. Mineralization of nitrogen did not occur until respiration had lowered the \arbon/nitrogen ratio to about 5 and mineralization of sulphur did not occur until respiration had lowered the carbon/sulphur ratio to about 50. On the other hand mineralization of phosphorus occurred before the carbon/phosphorus ratio had been reduced to any consistent figure. This may not be a characteristic of phosphorus mineralization but may have been caused by suboptimal supply of nitrogen.Mineralization of an element also depended on the concentration of other elements and, in general, reduced supplies of one element caused increased mineralization of others.