This type of generator was developed to meet the corrosion problems occurring in nuclear plants. Results are reported on the practical experience of 6 such generators, Oconee 1, 2, and 3, Arkansas 1, Three Mile Island and Rancho Seco, covering 27 to 43 mo operation. The supply water has total solid content of 50 ppb at most; the other characteristics such as dissolved O2, Si, Fe, Cu, Pb and N2H4 contents are given; the 6 reactors use various condensate systems (powd. resin, and deep bed) and different materials for the tubing (stainless steel, carbon steel, 90/10 Cu/Ni, and Admiralty brass). Failures in the tubes were rare, and occurred only in highly stressed components, although the actual mechanism causing the failures was not identified. The tubes are resistant to NaCl and OH- at high temperatures Investigations are being carried out into the reason for failures. The principle of once-through operation is considered sound.