We model the volumetric method commonly used in the measurement of gas-phase adsorption isotherms by using Monte Carlo (MC) simulation to study slit pore adsorption in a finite volume. Although the method has been used for a very long time, modeling of the operation by a Monte Carlo scheme to account properly for the exchange of mass between the solid and the finite dosing volume has not been widely studied in the literature. This paper presents the MC simulation of the system composed of the solid subsystem and the gas phase surrounding it. We show that not only the size of the dosing volume and the incremental dosing amount but also the pore shape, pore size, and temperature have significant effects on the unstable region of the phase diagram, especially when the system is going through a first-order transition. This study extends and augments the recent work of Puibasset et al.(1) by showing that the shape of the adsorbent walls and the incremental dosing amount can affect the chemical potential in the adsorption system.