The extended use of biomass for heat and power production has caused increased operational problems with fouling and high-temperature corrosion in boilers. These problems are mainly related to the presence of alkali chlorides (KCl and NaCl) at high concentrations in the flue gas. The in situ alkali chloride monitor (IACM) was developed by Vattenfall Research and Development AB for measuring the alkali chloride concentration in hot flue gases (less than or approximately 650 degrees C). The measurement technique is based on molecular differential absorption spectroscopy in the UV range. Simultaneous measurement of SO(2) concentration is also possible. The measuring range is 1-50 ppm for the sum of KCl and NaCl concentrations and 4-750 ppm for SO(2). This paper describes the principle of the IACM as well as its calibration. Furthermore, an example of its application in an industrial boiler is given.