Disclosed is a rector for thermally cracking monofunctional and/or polyfunctional carbamates into the corresponding isocyanate and hydroxyl components in the liquid phase, comprising devices which feed heat into the reactor. Said devices are embodied as heat exchange plates that are flooded with a heat-exchanging means. The geometry of the devices, which is characterized by the fact that the outgassing surface is proportional to the volume as well as by the arrangement of the heating surfaces, allows the cracking to take place in a two-phase mixture containing more than 50 percent by volume of gas.