Because of its excellent thermal-mechanical properties, diamond is a promising infrared window material. With the development of chemical vapor deposition (CVD) diamond technology, diamond windows and domes are becoming a practical reality. The infrared transmittance of type IIa and CVD diamond was characterized as a function of temperature, and the room-temperature ultraviolet transmittance of type IIa diamond was also measured. These experimental results were interpreted in terms of intrinsic and extrinsic lattice vibration models and the Urbach tail and weak absorption tail models. The first measurements of the temperature variation of the index of refraction in the 10-micrometers region for CVD diamond were obtained on a sample that showed strong modulation due to interference. Transmittance was investigated in most of the transparent range of diamond, although the 8- to 12-micrometers region is emphasized.