Gamma-ray spectra with energies up to 1.3 MeV have been detected for the first time with 1-cm-thick HgI/sub 2/ semiconductor detectors at room temperature. The spectra can be taken using a long, 10 ..mu..s, or a short, 0.5 ..mu..s, charge collection time. The latter has produced better results with better peak efficiency, higher peak-to-valley ratio, and fewer low energy counts with little sacrifice in resolution. Spectra for energies between 100 keV and 1.3 MeV and descriptions of the techniques for their acquisition are presented. The thick detectors need a conditioning treatment, which is described, prior to showing spectral response.