Data are presented which demonstrate that two different types of condensing instabilities can exist in the horizontal U-tube heat exchangers used in moisture separator reheaters. The first instability was a cyclic type. The thermocouple readings showed that at times the subcooled condensate temperatures leaving certain lubes were cycling in phase and with the same period. The maximum values of these temperature traces never reached the saturation temperature. The second instability was a condensate chugging or water hammer type. The thermocouple readings indicated that vapor was entering some lube ends because of a stratified flow regime. It was then condensed, and all the condensate was discharged as a liquid plug.