A detailed, segmented, highly visible cost comparison of the five leading stack gas desulfurization processes was conducted. Using data available in late 1973, complete economic evaluations were prepared for limestone slurry scrubbing, lime slurry scrubbing, magnesia slurry scrubbing - regeneration to sulfuric acid, sodium solution scrubbing - SO2 reduction to sulfur, and catalytic oxidation (Cat-Ox). Assuming the process technology to be proven in application, a prescribed set of representative power plant, process design, and economic premises was established. For each process design, projections are included for a base case (500-MW, 3.5% S in coal, new unit) and 16 other variations in power unit size, fuel type (coal or oil), sulfur in fuel, unit status (new vs. existing), solids disposal method (off-site vs. on-site ponding), and SO2 removal (80% vs. 90%). Capital investment, annual operating costs (7,000 hr/yr) and lifetime operating costs (over a 30-year declining operating profile) were estimated for the base case and each variation. Using sensitivity analysis, effects of variations in energy costs, raw material costs, maintenance costs, cost of capital, operating labor cost escalation, and net sales revenue were studied. A 3-year construction schedule ending in mid-1975 is assumed for a midwestern location. Investment costs (mid-1974 dollars) can be scaled or altered to reflect any predictable project schedule, escalation rate, or location. Definition of the systems estimated, sources of cost data, and recommended equipment size-cost scale factors are given.