Evaporative losses of adsorbed or absorbed species from particle deposits during sampling is discussed. The theory that is developed focuses on evaporative losses that occur as a result of the pressure drop within the sampling device. The theory assumes that temperature and gas and particle concentrations remain constant during sampling. When the atmospheric aerosol is in equilibrium, the evaporative losses from filter samples are predicted to be small. The losses from impactor samples may be large for impactor stages with large pressure drop, especially for chemical species that are predominantly in the gas phase. When vapor denuders are used upstream, predicted sampling efficiencies for filters are always poorer than for impactors. In all cases, sampling efficiencies decrease with increasing values of the equilibrium ratio of gas to particle concentrations of the evaporating species.