Hydroxyl (OH) content in diopside of one diamond-free ultrahigh-pressure (UHP) marble from the Kokchetav Massif, Kazakhstan, was estimated by a micro–Fourier transform infrared (FTIR) spectrometer. Two intrinsic diopside OH bands of FTIR absorption spectra were identified at 3645 and 3465 cm⁻¹, with the former band dominating the spectra. Another band occurred at 3685 cm⁻¹, which seemed to be accompanied by trivial bands at 3750, 3730, 3585, and 3565 cm⁻¹. Intensity variation of the 3685 cm⁻¹ band within a single diopside crystal and its location may suggest that the cause of this band was...