Process for detecting a gaseous compound of BX3, HX or X2 type within a gas using a composition containing a probe molecule, characterized in that the probe molecule is a molecule for which the reaction with one or more compounds of BX3, HX or X2 type leads to a variation of at least one of its physicochemical properties, this variation being measurable via a suitable analysis technique, and in that the following steps are carried out in this order: (a) measurement of said physicochemical property of the probe molecule, such as a spectral property, (b) bringing the gas into contact with the composition containing the probe molecule from step (a), (c) repeat measurement of said physicochemical property, (d) correlation of the variation of said spectral property between steps (a) and (c) in the presence of said gaseous compound of BX3, HX or X2 type, the measurement of the physicochemical property from step (a) possibly being a prior step, process for trapping gaseous compounds of BX3, HX or X2 type contained in a gas, material capable of reacting with at least one compound of BX3, HX or X2 type in gaseous form and sensor for compounds of BX3, HX or X2 type.