Using adiabatic invariance and the Bohr—Sommerfeld quantization rule we investigate the entropy spectroscopy of two black holes of heterotic string theory, the charged GMGHS and the rotating Sen solutions. It is shown that the entropy spectrum is equally spaced in both cases, identically to the spectrum obtained before for Schwarzschild, Reissner—Nordström and Kerr black holes. Since the adiabatic invariance method does not use quasinormal mode analysis, there is no need to impose the small charge or small angular momentum limits and there is no confusion on whether the real part or the imaginary part of the modes is responsible for the entropy spectrum.