Chlorine is an important industrial chemical. Not only is it a component of many important products, it is also needed for many chemical manufacturing processes, even where it does not appear in the final product. But a number of chlorine chemicals, especially organochlorines, are toxic, carcinogenic, tentogenic or otherwise potentially disturbing to the environment. For this reason, some environmentalists—notably Greenpeace‐have advocated a ban, not just on some products but on all uses of elemental chlorine. The chemical industry is taking this threat seriously and mounting a vigorous defense. But the debate so far is not illuminating the issues effectively, because both sides are selectively using questionable and unverifiable data.The scientific uncertainties are not really the problem. Rather, data in the public domain and accessible to environmentalists and even regulatory authorities are of very poor quality. Because of industry secrecy much crucial information is unavailable and some of what is available is misleading or wrong. The dual purposes of this article, and the ones that follow, are (I) to elucidate the information requirements for an adequate life‐cycle analysis of chlorine and its uses and (2) to indicate how and where the use of massbalance methodology can help identify errors and fill in gaps. The present article deals with electrolytic chlorine production and mercury flows arising from chlorine production. Subsequent articles deal with conversion processes and losses and further chemical industry uses of chlorine, major end uses of chlorine and chlorine chemicals, and persistent organochlorine pollutants.