Numerous requests for information regarding oxygen demand and dissolved oxygen tests have indicated the desirability of assembling this scattered material and of placing it in a more readily usable form. In the instructions given herewith for the conduct of the oxygen demand test, the only notable departure from previous recommendations is the substitution, under certain conditions, of a buffered synthetic dilution water for stored tap water. This change appears desirable in view of the difficulty experienced in many places in securing a suitable dilution water, owing to the presence of unusual amounts of iron in the tap water supply or to the undesirable pH value, at times, of softened water or of water purified by excess-lime treatment.