A review of the available data on sulfate emissions methods and emissions rates from both catalyst-equipped and noncatalyst cars has been made. Air-dilution methods of various sorts appear to give similar sulfates emissions results, comparable with those obtained by controlled condensation procedures. Mobile source SO2 measurements technology requires much more attention. At present, only the hydrogen peroxide oxidation to sulfate has been demonstrated to give reliable results. Summaries of several hundred catalyst and non-catalyst emission rates are treated to estimate California and 49-state emissions. A cruise-mode emission rate of about 0.03 g/mile appears appropriate for both monolithic and pelleted catalysts in 49-state cars.