Difference in the macrostructure of vapor‐deposited B and SiC filaments have been related to the extent of reaction between the deposited material and its tungsten substrate. Chemical reactions between W and B during deposition create mechanical dilatations of sufficient magnitude to produce radial cracks in the B sheath running the length of the filament. Similar reactions between W and SiC do occur but such cracks do not form because both the amount of the reactant is less and the corresponding volume expansion is significantly smaller.