This paper presents computed dependencies of the detected electron energy distribution on the water vapour pressure in an environmental scanning electron microscope obtained using the EOD software with a Monte Carlo plug-in for the electron-gas interactions. The software GEANT was used for the Monte Carlo simulations of the beam-sample interactions and the signal electron emission from the sample into the gaseous environment. The simulations were carried out for selected energies of the signal electrons collected by two electrodes with two different diameters with the voltages of +350 V and 0, respectively, and then 0 and +350 V, respectively, and for the distance of 2 mm between the sample and the detection electrodes of the ionization detector. The simulated results are verified by experimental measurements. Consequences of the simulated and experimental dependencies on the acquisition of the topographical or material contrasts using our ionization detector equipped with segmented detection electrode are described and discussed.