Chlorine is manufactured by the electrolysis of brine. The chlorine product is a gas, which is collected, dried, compressed, and cooled to produce a liquid. This paper describes the development and field demonstration of a membrane process to recover chlorine from the liquefaction tail gas of chloralkali plants. The tail gas consists of about 20% chlorine and 50-70% air, with the balance being hydrogen and carbon dioxide. A number of membrane materials can achieve a selectivity of 20 or more for chlorine from nitrogen, but degradation of the membrane materials in the presence of high concentrations of chlorine gas often occurs. However, modified silicone rubber membranes are stable to chlorine gas streams. Silicone rubber composite membranes were prepared and formed into modules of 1-2 m(2) membrane area. The modules were tested in the laboratory and in a field test on a slip stream from a chlorine liquefaction unit. In the laboratory, chlorine/nitrogen membrane selectivities of more than 40 were obtained, but selectivities of 6-10 were measured in the field test. This decrease in selectivity was caused by low gas flow rates through the modules, which resulted in concentration polarization effects. However, the membrane maintained essentially constant fluxes and selectivities in field tests lasting more than 1 month. Calculated plant designs based on a selectivity of 8 are able to recover more than 95% of the chlorine in the tail. gas. Typical project payback times based on the value of the recovered chlorine and avoided caustic scrubber chemical use are expected to be 1-2 years.