The permeation resistance of chlorinated polyethylene (CPE) used in totally encapsulating chemical protective suits against the aerospace fuels hydrazine, monomethylhydrazine, and uns-dimethylhydrazine was determined by measuring the breakthrough time and time-averaged vapor transmission rate (VTR) using procedures consistent with ASTM Test Methods for Resistance of Protective Clothing to Permeation by Liquids or Gases Under Conditions of Continuous Contact (F 739) and Intermittent Contact (F 1383). Two exposure scenarios were simulated: a 2-hour (h) fuel vapor exposure, and a liquid fuel “splash” followed by a 2-h vapor exposure. To simulate internal suit pressure during operation, a positive differential pressure of 0.3 in. water (75 Pa) on the collection side of the permeation apparatus was used. A model was developed using the available data to estimate propellant concentrations inside an air-line fed, totally encapsulating chemical protective suit. Concentrations were calculated under simulated conditions of fixed vapor transmission rate, variable breathing airflow rate, and variable splash exposure area. Calculations showed that the maximum allowable permeation rates of hydrazine fuels through CPE were of the order of 0.10 ng cm-2 min-1 for encapsulating suits with breathing airflow rates of 5 to 9 scfm (140 to 255 L min-1). Above these permeation rates, the 10 parts per billion (ppb) threshold limit value-time weighted average could be exceeded. To evaluate suit performance at ppb-level concentrations, use of a sensitive analytical method such as cation exchange high performance liquid chromatography with amperometric detection, was found to be essential. The analytical detection limit determined the lowest measurable VTR, which in turn governed the lowest permeant concentration that could be calculated inside the totally encapsulating chemical protective suit.