One of the easiest ways to maintain a reaction mixture at an approximately constant temperature is to heat the mixture until it boils and then condense the vapors and return the condense to the reaction flask. This process, which is quite common and very important, is known as heating under flux, or just refluxing. The temperature at which to perform the reaction can be chosen merely by selection of a solvent with the appropriate boiling point.

It is essential that the top of the reflux condenser is left open to the atmosphere; otherwise pressure would build up in the apparatus until an explosion occurred.

The temperature of the reaction mixture remains the same regardless of how fast boiling is taking place; the rate of refluxing should be such that the vapors are condensed in the lower one-half to two-thirds of the reflux condenser. A few wisps of vapor may occasionally except out the top of the condenser but these should generally be disregarded. Some reaction may, of course, evolve hydrogen chloride, carbon dioxide, or some other gas that will except out the top of the condenser; but this is usually desirable. Occasionally if a very volatile solvent, such as diethyl ether, is refluxed, two or more reflux condensers stacked on top of each other may be employed to minimize loss of the solvent.

Reflux periods for reactions may vary from a few minutes to a few hours or even to several days. Timing of reflux periods commences only after the mixture is boiling at the proper rate. Although refluxing systems require only a minimum of attention, they should rarely be left entirely unattended for more than five minutes.