The PM_2.5 speciation trends network (STN), part of the Chemical Speciation Network (CSN), has been operational since the year 2000.  PM_2.5 samples are collected on Teflon, Nylon and Quartz filters.  Teflon filters are used to determine gravimetric mass and analyzed for elemental composition using XRF.  Nylon filters are analyzed for ions using ion chromatography, and Quartz filters are analyzed for organic and elemental carbon using thermo-optical analysis.  In this work, we compare elemental sulfur concentrations measured on Teflon filters with XRF to sulfate ion concentrations found on Nylon filters.  If all of sulfur is present in the form of sulfate, the sulfur to sulfate ratio would be equal to 0.33.  Alternatively, three times that ratio will equal 1.0.  These ratios were calculated in data from all CSN sites from 2003 through 2013. The data were stratified by year and season for the entire network and for each site to examine spatial and temporal patterns.  The sulfur and sulfate data were highly correlated, with the exception of a few outliers.  A seasonal pattern in the distribution of the ratio was observed, with higher values during the summer during some years.  This suggests that the non-sulfate sulfur may be associated, at least partly, with secondary organo-sulfate compounds. Confirmation of the presence of excess sulfur in water soluble forms was verified by selecting a small set of samples with ratios exceeding our current control limits and reanalyzing these for sulfate with and without an additional oxidation step by ion chromatography.