Abstract: Ceramic heat exchangers having narrow cross-sections are sealed by filling one side (which later will be the cold side) of the heat exchanger with a liquid synthetic resin and generating there a pressure higher than that on the other side (which later will be the hot side) of the heat exchanger, removing the excess resin and fully curing the synthetic resin. The process is suitable especially for ceramic heat exchangers of silicon nitride. In this case, a suspension of finely dispersed abrasive in a liquid phase can be pumped, before the treatment with the liquid synthetic resin, through that side of the heat exchanger which will later by the cold side, until the silicon nitride whiskers formed on reaction firing have been mechanically removed, and the heat exchanger can then be rinsed and dried.