Abstract The optical absorption edge of CuGa(S x Se 1− x ) 2 single crystals is measured as a function of hydrostatic pressure, covering the full stability range (0 to 30GPa) of the chalcopyrile phase. For 0 ⩽ x ⩽ 1, a linear increase of the direct energy gap with pressure is found. The corresponding gap deformation potential is calculated to be −4.5 eV for x = 0, −5.2 eV for x = 0.5, and −4.4 eV for x = 1. From changes in the light transmission characteristic of the samples under pressure, structural first‐order phase transitions are observed.