Global winds provided by satellite scatterometry are an important aspect of the ocean observing system. Many applications require well-calibrated time series of winds over time periods spanned by multiple missions. But sensors on individual satellites differ, introducing differences in wind estimates. This study focuses on global winds from two scatterometers, ERS-2 (1996-2001) and QuikSCAT (1999-2009) that show persistent differences during their period of overlap (July-1999 to January 2001). We examine a set of collocated observations during this period to evaluate the causes of these differences. The use of different operating frequencies leads to differences that depend on rain rate, wind velocity, and SST. The enhanced sensitivity to rain rate of the higher frequency QuikSCAT is mitigated by a combined use of the standard rain flag and removing data for which the multidimensional rain probability is >0.05. Generally, ERS-2 wind speeds computed using the IFREMER CMODIFR2 geophysical model function (GMF) are lower than QuikSCAT winds by 0.6 m/s, but wind directions are consistent. This wind speed bias is reduced to -0.2 m/s after partial reprocessing of ERS-2 wind speed using Hersbach's (2010) new CMOD5.n GMF, without altering wind direction. An additional contributor to the difference in wind speed is the biases in the GMFs used in processing the two data sets and is empirically parameterized here as a function of ERS-2 wind speed and direction relative to the mid-beam azimuth. After applying the above corrections, QuikSCAT wind speed then remains systematically lower (by 0.5 m/s) than ERS-2 over regions of very cold SST<5 degrees C. This difference may result from temperature-dependence in the viscous damping of surface waves which has a stronger impact on shorter waves and thus preferentially affects QuikSCAT. (C) 2013 Elsevier B.V. All rights reserved.