Dielectric stack mirrors consisting of alternating layers of Ga1-xAlxAs/Ga1-yAlyAs, grown by solid source molecular beam epitaxy, are evaluated using reflection spectroscopy and a complex index of refraction model. The dielectric stack mirrors have asymmetric reflectivity spectra in the near infrared. We contend that this asymmetry is due to loss : in the mirror stack and propose that the loss may be caused by defects introduced during the growth process. These reflectivity spectra are compared to a model that includes loss by utilizing complex indices of refraction. Calculated spectra reproduce this asymmetry using small imaginary index components. This study demonstrates the validity of using a theory that includes loss for modeling the reflectivity spectra of a dielectric stack mirror in the spectral range of 0.7 to 0.9 mu m.