A system has been designed to evaluate MPC level concentrations of 131I in reactor environments. The system provides reliable determinations of total 131I concentrations even under such unfavorable sampling conditions as high relative humidity or very low concentrations. Furthermore, the fraction of methyl iodide present can also be determined from the activity distribution on the several parts of the system. Field experiments were conducted at Consolidated Edison's Indian Point No. 1 reactor. The results of these tests are presented and discussed.