With the increasing use of bactericidal ultraviolet lamps it has become very desirable to have more information about the accompanying production of ozone which is the only feasible agent for protecting surfaces not exposed to direct or scattered radiation. Very little published information is available and practically all the data given in this article are new. A low pressure mercury arc lamp of the type of the Westinghouse Sterilamp gives an amount of ozone of the order of 1∕100th gram per hour, corresponding to a rate of emission of energy of at least 1∕100th of a watt. A gentle air motion increases the yield. All the ozone is produced within a few feet of the lamp. The rate of decomposition of the ozone after the lamps are turned off is faster the greater the surfaces compared with the volume, the higher the temperature, the higher the humidity, and the more 2537 radiation present. The most important factor, the equilibrium ozone concentration, in an enclosure of several cubic meters is proportional to the ozone produced, inversely proportional to the volume, and is reduced to about one‐third when the relative humidity changes from a few percent to near saturation and to about one‐half when the temperature is raised from 7° to 32°C. It is very roughly 25 percent lower if the 2537 radiation of the present Sterilamp, accompanying the short wave‐length ozone radiation, is doubled.