Optical and scanning electron microscopic examinations were made of the solids in aqueous suspensions of crushed limestone that had been used to scrub sulfur oxides from power plant stack gas. Most of the sulfur was recovered as CaSO3 0.5H2O which formed small thin plates that settled slowly; the relatively small amount of gypsum, CaSO4 · 2H2O, that was formed was present as massive crystals that settled rapidly. Scaling was encountered in significant amount only on the mist eliminator which had been washed periodically with the supernatant solution from which the solids had been settled. The scale was composed principally of massive gypsum crystals, but the plastic surfaces on which scale had formed were covered with a fine-grained mixture of fly ash and calcium sulfite that acted as the nucleating surface for the gypsum. The fly ash in the scrubber solids was present mostly as spheres, and these acted only as inert diluents of the solids.