Portable kit for measuring the hydrogen sulfide concentration of a gas likely to contain it, said kit comprising separate modules that can be connected to one another, said modules being the following: - a measurement module A comprising a measurement chamber M wherein the absorption by the gas of a monochromatic electromagnetic radiation is measured; - an expansion module B that makes it possible to bring the pressure of the gas to be analysed to the working pressure of the measurement module; - a pressure-regulating module C capable of maintaining the pressure of the gas in the measurement chamber at a value lying within the range of working pressure values of the measurement module; - a module D for processing the absorption measurement, which makes it possible to obtain the concentration of hydrogen sulfide in the gas, and also - means which make it possible to connect the modules to one another, said kit further having the following characteristics: - the expansion module B has an inlet duct that receives the gas to be analysed and an outlet duct connected to an inlet duct of the measurement module A; - the measurement module A is electrically connected to the processing module D; and - the regulating module C is mechanically connected to the measurement module A (5, 7) and to a duct for discharging the gas out of the kit; said kit satisfying point "p" (suppression of the explosive atmosphere) and/or point "e" (suppression of the source of combustion) and/or point "d" (explosion-proof, no propagation of combustion) of the ATEX classification. - a measurement module A comprising a measurement chamber M wherein the absorption by the gas of a monochromatic electromagnetic radiation is measured; - an expansion module B that makes it possible to bring the pressure of the gas to be analysed to the working pressure of the measurement module; - a pressure-regulating module C capable of maintaining the pressure of the gas in the measurement chamber at a value lying within the range of working pressure values of the measurement module; - a module D for processing the absorption measurement, which makes it possible to obtain the concentration of hydrogen sulfide in the gas, and also - means which make it possible to connect the modules to one another, said kit further having the following characteristics: - the expansion module B has an inlet duct that receives the gas to be analysed and an outlet duct connected to an inlet duct of the measurement module A; - the measurement module A is electrically connected to the processing module D; and - the regulating module C is mechanically connected to the measurement module A (5, 7) and to a duct for discharging the gas out of the kit; - the expansion module B has an inlet duct that receives the gas to be analysed and an outlet duct connected to an inlet duct of the measurement module A; - the measurement module A is electrically connected to the processing module D; and - the regulating module C is mechanically connected to the measurement module A (5, 7) and to a duct for discharging the gas out of the kit; said kit satisfying point "p" (suppression of the explosive atmosphere) and/or point "e" (suppression of the source of combustion) and/or point "d" (explosion-proof, no propagation of combustion) of the ATEX classification.