Traditional phase change materials can store latent heat with high energy density providing they are stored above the phase transition temperature. In contrast, photochemical phase change materials allow latent heat storage at, or below, ambient temperatures and heat release can be triggered using heat or light stimuli. Herein, we report that crystalline binary mixtures of azobenzene and 4-methoxyazobenzene form photochemical phase change materials that possess working temperatures in the range of −58 °C to 31 °C. These mixtures can store and release up to 266 J g⁻¹ of latent heat energy.