Abstract: An arrangement for monitoring the concentration of potentially explosive substances in gas streams includes a first flame-ionizer and a second flame-ionizer, both ionizers being accommodated in a common chamber which is maintained at a predetermined temperature, such as 180.degree. C. A common conduit for supplying both ionizers with a fuel gas is provided in the arrangement. A common conduit for feeding both ionizers with combustion air is mounted in the arrangement. Each ionizer is provided with an individual conduit connecting the assigned ionizer with a respective drying chamber of a coating installation. A sample gas stream discharged from the respective drying chamber is fed into the assigned ionizer through a capillary pipe and passed through a flame produced in each ionizer. The electric current flowing between the electrodes in each ionizer shows the concentration of potentially explosive substance in gas streams being tested.