Total electron-yield (TEY) detection has long been regarded a method for XAS investigations of surface adsorbates under ultra-high vacuum conditions. Recent work has shown, however, that the technique can also be applied to in situ studies of catalysts under reaction conditions. We have thoroughly investigated the TEY signal formation process under gas-flow conditions with a view to (i) characterizing the surface sensitivity of the technique, (ii) assessing the accuracy of the information acquired, and (iii) establishing the practical possibilities and limitations of true in situ measurements at elevated pressures and temperatures. We conclude that the technique opens up new possibilities particularly for XAS investigations of unsupported and/or concentrated materials that were hitherto difficult to study by other XAS detection techniques. In situ TEY detection is most promising for studies with medium and hard X-rays (E > 3 keV), which penetrate the gaseous environment of catalytic reactors most readily. Data are presented for iodine monolayers on Au(111): these show that monolayer studies are, in principle, possible, but represent the present limit of the technique in the medium and hard X-ray ranges. Results for a Cu-based catalyst for the synthesis of methanol are discussed to demonstrate what information can be achieved by the technique.