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Abstract: During January 2006, a new hybrid PM-type in-vacuum mini-gap undulator (MGU) with NdFeB magnets and vanadium permendur poles was installed within the X25 straight-section of the 2.8GeV National Synchrotron Light Source (NSLS) electron storage ring. This new insertion device was implemented to deliver high brightness X-ray beams within the energy range of ∼5.5–20keV for monochromatic single-crystal X-ray macromolecular crystallography. The spectral output from such a device is very sensitive to the storage ring machine parameters and in this paper, we report the precise determination of the new X25 MGU performance by implementing a single-crystal silicon (111) X-ray spectrometer to measure the X-ray energy-dependent flux throughput and infer the corresponding brightness values for a wide range of undulator gap openings. We compare these values to synchrotron radiation calculations using the SPECTRA (version 8.0) code and confirm both the integrity of the new X25 MGU device and the emittance values of the NSLS electron storage ring. [Copyright &y& Elsevier]