In this work, we explore possible topological insulating state in bismuth doped with arsenic . Infrared and magneto‐optical spectroscopy are employed to probe the electrodynamic response of with x = 0.01, as well as topological insulator Sb with x = 0.20. The spectra are reported in magnetic fields up to 18 T, and at temperatures between 10 and 300 K. The results indicate strong sensitivity of optical properties to these external stimuli in both and , but also some differences introduced by arsenic doping. Most notably, the field dependence of cyclotron resonance in implies that it is due to bulk carriers, as opposed to surface carriers in .