The main goal of this chapter is to deal with a study of the thermal resistance of barrier materials of the Fire Rescue Service anti-gas isolative garments Trellchem Light and Tychem TK. These garments are used in the conditions of the Czech Republic Fire Rescue Service chemical service specialists’ protection against the effects of toxic compounds, including chemical warfare agents. The thermal analysis of samples was performed using the differential scanning calorimetry, the surface structural changes were determined via the 3D optical profilometer, and the gas (nitrogen) permeation measurements were performed using the integral permeation method. The appropriate time limits and the exposure temperatures during which the anti-gas chemical protective garments could be securely used were evaluated based on permeation results. In this chapter, theoretical and practical approaches to assessing the gas permeability of barrier materials used to construct protective garments of an isolative type designated for body surface protection of the chemical service of the Fire Rescue Service have been studied.