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Synchrotron radiation (SR) is a fundamental tool for X-ray research. In particular, X-ray absorption spectroscopy (XAS) accesses information as electronic properties, local structure or chemical-physical state in condensed-matter studies. Ionization chambers (ICs) are the most widely used XAS detectors for transmission measurements because of their reliability, high linearity and good stability. Recently, solid-state detectors have been considered and Si p-i-n has been applied to high fluxes (1011 ph/s), where the linearity of ICs is no longer guaranteed. Silicon photodiodes exhibit an extremely linear response in at least 5 decades but show diffraction peaks. Diamond is an ideal substrate to produce radiation-hard, low dark current (<1 pA/cm2), visible-blind and fast-response X-ray detectors with a high S/N ratio. Diamond detectors were tested as SR monitor capable to withstand the high photon flux density of the 3rd generation SR sources. Being the lowest X-ray-absorbing solid-state dielectric material, diamond maximizes the flux through thin self-standing devices with minimal spectral effect down to the soft x-ray range. We will present results of X-ray tests of photoconductors based on different diamond substrates. The results will be compared to standard ICs for XAS applications in terms of spectral quality, noise and linearity in the 4–13 keV energy range. [ABSTRACT FROM AUTHOR]