Characterization of room temperature Fourier transform infrared spectro- scopy for MBE-grown Ga_(1-x)In_xAs_(1-y)Sb_y on (100) semi-insulating GaAs and (100) Te-doped GaSb substrates has been studied and the causes for the difference between the resulting spectrum of GaInAsSb/GaAs and that of GaInAsSb/GaSb have been successfully interpreted. The band gap energies of GaInAsSb have been determined ex- perimentally and compared with those obtained by an interpolation method. It is found that for all samples investigated, the experimental data are slightly larger than the theo- retical ones, of which the origin is discussed in detail in this paper.