Chlorine is produced by reacting hydrogen chloride and oxygen in the presence of a chromium oxide catalyst in a reactor whose catalyst-contacting part is lined with one of lining materials represented by the following general formula (I):M.sub.a X.sub.b (I)wherein M can be boron, aluminum, silicon, titanium, zirconium or chromium, X denotes oxygen, nitrogen or carbon, a is an integer of 1-2 and b stands for an integer of 1-3 or with a mixture of at least two of the lining materials. The above process and reactor are effective in maintaining the activity of the catalyst.