Ions in the ion mobility spectrometry (IMS) are mostly hydrated. A single peak in the drift time spectrum is usually generated by a mixture of ions differing in the number of attached water molecules. Under real IMS detector operating conditions, ions change their composition during movement in the drift region due to the changes in the number of water molecules attached to the ion. The impact of water vapor on the drift times of small ions at different temperatures was studied experimentally using an ion mobility spectrometer. The experiments were carried out for hydronium, ammonium, oxygen, chloride, bromide, and iodide ions. A theoretical model was developed, allowing us to calculate the effective mobility of ions for a given concentration of water vapor and temperature. The basic assumption adopted in this model was the linear dependence of the effective mobility coefficient on the mobility of ions with a certain degree of hydration. The weighting factors in this relationship are the abundances of individual types of ions. These parameters were determined by calculations based on the thermodynamics of the formation and disintegration of ionic clusters. From the known values of temperature, pressure, and humidity, the values of effective mobilities can be predicted quite accurately. The dependencies of reduced mobilities on the average degree of hydration were also determined. For these dependencies, the measurement points on the graphs are gathered along specific lines. This means that the average degree of hydration unambiguously determines the value of reduced mobility for a given type of ions.