A theoretical investigation is conducted for the first time to explore the deliquescence of particles deposited on a substrate. The formulation incorporates the Kelvin effect with the assumption that the dry and wet particles are both spherical caps in shape. Unlike the deposited particles larger than 500 nm, the deliquescence relative humidity (DRH) of smaller particles can substantially depend on the particle size, the contact angles, and the surface tension between the particle and the atmosphere. At certain contact angles, small particles depositing on a substrate could deliquesce at a much lower RH, posing a potential corrosion problem for metallic substrates.