Ice core from Greenland was melted, filtered, and homogenized. Its stable hydrogen (δ2H) and oxygen (δ18O) isotopic compositions were adjusted to approximately match those of USGS46 ice core water (δ2H = –235.8 ‰ and δ18O = –29.80 ‰). This isotopic reference water, USGS46a, was loaded into glass ampules, sealed, autoclaved to eliminate biological activity, and calibrated by dual-inlet isotope-ratio mass spectrometry and laser absorption spectrometry. USGS46a is intended as one of two secondary isotopic reference waters for daily normalization of stable hydrogen and stable oxygen isotopic analysis of water with a mass spectrometer or a laser absorption spectrometer. The measured δ2H and δ18O values of USGS46a reference water were –235.6 ± 0.7 ‰ and –30.09 ± 0.05 ‰, respectively, relative to VSMOW on scales normalized such that the δ2H and δ18O values of SLAP reference water are, respectively, –428 and –55.5 ‰. Each uncertainty is an estimated expanded uncertainty (U = 2uc) about the reference value that provides an interval that has about a 95-percent probability of encompassing the true value. This reference water is available in cases containing 144 glass ampules that are filled with 5 mL of water per ampule and in silver tubes for use in high temperature elemental analyzers.