The Mg II core-to-wing index is a ratio of the Mg II chromospheric emission at 280 nm to the photospheric radiation in the line wings and is used as an indicator of solar activity. Since October 1991, the Solar--Stellar Irradiance Comparison Experiment (SOLSTICE) has made daily irradiance measurements in the range 119--420 nm from the Upper Atmosphere Research Satellite (UARS). A new Mg II index, based on the SOLSTICE observations at a spectral resolution of 0.24 nm, is presented and compared to previous measurements. Spectral irradiance measurements of the Mg II doublet at low spectral resolution (≈1 nm) have been made by the Solar Backscatter UltraViolet (SBUV) instrument on Nimbus 7 since November 1978 and subsequently by the SBUV2 instruments on NOAA 9 and NOAA 11 satellites. We compare the SOLSTICE data with the Mg II time series derived from SBUV2 data by the groups at the National Oceanic and Atmospheric Administration (NOAA) and at the Goddard Space Flight Center (GSFC). SOLSTICE data are convolved to the lower SBUV2 resolution, and the NOAA and GSFC algorithms are then applied to this data set. The SOLSTICE Mg II indices constructed in this manner simulate the SBUV2 indices and can be used to validate the SBUV2 time series and identify data problems. From our analysis, we conclude that the NOAA Mg II time series is the most consistent during the period 1978--1994. The new GSFC Mg II time series has comparable accuracy for the period starting in 1989. We also derive the linear transformation equations required to put the high- and low-resolution time series onto common scales.¿ 1997 American Geophysical Union