Size distribution is a critical parameter that determines the dynamics, transport and deposition of aerosol particles in a given system. Scanning Mobility Particle Sizer (SMPS), based on the principle of particle mobility classification in an electric field is one of the commonly used instruments for measurement of size distribution. To maintain laminarity of aerosol particles, a sheath air flow is generally provided annular to the sampled aerosol particle flow. Available literature suggests drying of the sheath air to avoid effect on charge neutralisation of the sampled particles and to safeguard the central high voltage electrode. However, there are certain ambiguities associated with the use of dried sheath air for SMPS measurements. This study investigates the effect of sheath air drying on particle size distribution measurements. It focuses on experiments performed with substrates of differing hydrophilic properties (e.g., NaCI, NaNO3 and nichrome) at different relative humidity (RH) levels. Results indicated a prominent effect of sheath air drying dependent on RH and material properties. For NaCI aerosol particles, a much lowered (by a factor of 2) mean size was observed at higher RH level with SMPS using a sheath air drier. The results indicate that the effect of sheath air drying on SMPS measurements should be clearly included in algorithms and protocols when interpreting the measurements.