Intense treatment of feed water solves high demands on a demineralized water quality in the processes involving hot water/steam boilers. There are various demands on water quality parameters - its hardness, alkalinity, pH value, carbon dioxide and oxygen content, etc. -according to the type of boiler and its working pressure. Collectively, efficient demineralization and/or softening are always inevitable. Every desalination water treatment unit consists of a standard pre-treatment part and a demineralization part. Arrangement of the pre-treatment part depends on the kind of water source (well, surface, or tap water) and its individual analysis. This part of a water treatment unit is essential to protect the plant. Thereafter, the demineralization part of a water treatment unit is chosen and designed to meet product water quality demands. There are two basic methods used for brackish water desalination in the Czech Republic: reverse osmosis and ion exchangers. An application is presented of the Mega Company's reverse osmosis units in the area of water treatment for boilers. It describes some installations and their properties - source water quality, pre-treatment techniques, and product quality. A brief comparison of the reverse osmosis process and ion exchangers is also presented.