Ion mobility spectroscopy is used to identify chemical species, especially narcotics and explosives. This technique is based on ion mobility which depends on temperature, pressure, ion nature, and molecules in which the ion moves. Peaks are displaced by the temperature and the pressure. For each material , at certain temperature and pressure, there are certain characteristic peaks that can be used to identify the material s . The moisture content of the gas also has a significant effect on the displacement of the peaks, which can lead to an error in the identification of the materials. These studies have been carried out on reactant species in IMS Research Laboratory of Isfahan University of Technology. In this research, the effect of water vapor on drift time of product species has been investigated. For this purpose, two kinds of materials, narcotics and explosives, were selected. The ionization source used in this project was Corona. Because of the high proton affinity of narcotics, positive Corona has been used, and the reactant ion selected for this group of materials was ammonium. Also Due to the high electron affinity of explosives, we used negative Corona, and also chlorine ion as the reactant ion. The experiments were carried out at three different temperatures: 200, 180 and 160 degrees Celsius. The moisture, saturated water vapor, was introduced to the system in intervals of 10 cm 3 /min from 0 to 100 cm 3 /min and through the drift gas. The overall humidity, the total humidity of the environment and the added water vapor, was measured by the water vapor concentration monitor in ppm. In dry condition where the amount of steam added through the gas to the tubes was zero, the moisture content is 12 ppm. By adding moisture, the peaks moved toward the drift time. Our results show ed that the inverse of the drift time changes linearly with the logarithm of water concentration. This behavior is similar to the behavior observed for reactant ions. However, in this case the slope is less and there are no significant changes for heavy ions. The reason for this observation is the lack of absorption of water molecules by massive heavy ions. Applying the results of our project, it is possible to predict and correct the displacement of peaks due to environmental humidity penetration and minimize identification error.