Polystyrene microshells filled with deuterium and tritium gas are important target shells for inertially confined fusion (ICF) and are particularly promising for targets containing spin–polarized hydrogens fuels. A currently active approach to the latter uses polarized D in HD, in a method which requires preservation of the high purity of the initially prepared HD (very low specified H2and D2concentrations). This would not be possible if dissociation should occur during permeation into the target shells. We have thus tested polystyrene shells using a novel method which employs very pure ortho‐D2as the test gas. An upper limit of 6×10−4was deduced for the dissociation probability of D2upon room temperature permeation through an approximately 8 μm wall of polystyrene, clearing the way for use of polystyrene target shells for ICF fusion experiments with spin–polarized hydrogen–isotope fuels.