The monolayer adsorption capacity of catalysts is usually determined ex situ, i.e. by physical adsorption of N-2 at 77 K via the BET method in a set-up separate from the reactor used for characterization or catalytic tests. We present here a new volumetric method working in the dynamic regime and using the physisorption of Ar at 77 K in the same microreactor where other characterization experiments are performed. The method allows the in situ measurement of the total surface area via a three-component gas system. The use of a third gas, as opposed to the two-component system used in commercial devices, allows us to take into account the considerable changes of the total volumetric flow rate associated with temperature variations. A mass spectrometer is employed as analytical device. The method is validated by comparing the monolayer capacity for Ar and N-2 on the same sample of Acrosil 200; the derived value of the Ar coverage cross-section is 19.2 Angstrom(2). Using the same method, we measure the surface areas of Co-Cu-based catalysts during the various temperature-programmed pretreatments of their oxalate precursors, Results show that the surface area can change by a factor as large as 100 depending on the pretreatment.