Optical properties of new promising nonlinear mixed-type crystals like AgGaxIn1–хSe2 are studied. Absorption coefficients and refractive indices in the IR (0.8 to 18 μm) and submillimeter (400–2000 μm) spectral regions are determined. Coefficients of Sellmeier's dispersion equations for the crystals under consideration at different In content are found for the first time. Phase-matching diagrams are calculated and plotted for the three-frequency interaction. The influence of indium percentage on the occurrence of phase-matching conditions are studied for different schemes: optical parametric oscillators pumped with YAG:Ho3+ (λ= 2.1 μm) and YAG:Er3+ (λ= 2.94 μm) lasers, СО2-laser second harmonic generation, and СО2-laser difference-frequency generation in the submillimeter region.