Utility boilers fired with high-calcium coals from the Western United States often develop massive ash-fouling deposits on steam tubes at much lower temperatures than is observed when low-calcium coals are burned, In order to define and develop the capability to predict the problems, extensive sampling was performed at five utility boilers, including both pulverized coal- and cyclone-fired units, burning a total of four different high-calcium subbituminous coals. Four types of low-temperature deposits were observed, three types forming on the upstream side of the tubes and one type forming on the downstream side, Upstream deposits include massive ones that form toward the back of the secondary superheater and into the reheater at gas temperatures between approximately 1700 degrees and 1900 degrees F, enamel-like deposits that form at lower temperatures on primary superheaters and economizers, and double-crested deposits that form in the primary superheater and economizer tubes. The deposits forming on the downstream sides of the tube occur from the reheater section to the economizer, The common and defining characteristic of the four types of low-temperature deposit is their high concentrations of sulfur that was fixed through chemical vapor deposition after the ash had deposited, In this article the different types of low-temperature fouling problems associated with the combustion of high-calcium coals are defined and the boiler conditions under which they occur are given. In subsequent articles the specific phenomena affecting the formation of the deposits will be addressed in detail, including the particular fractions of the ash responsible, the mechanisms of transport to the heat-exchanger surfaces, and the factors affecting the rates of deposition and development of strength in the deposits.