The hygroscopic growth of NaCl particles was investigated both experimentally and theoretically. Straightforward experiments done with a humidity controlled microbalance show that a macroscopic time (hours to days) is required before the final (equilibrium) size is reached. The developed model shows that the droplets are not homogeneous salt solutions and that a finite liquid phase diffusion resistance exists. This is clearly evident from the experimental data also: the initial stage of growth shows a linear mass growth in time. This would not be possible without the liquid-phase resistance. It was also possible to determine the liquid-phase diffusion coefficient. Although the value depends on composition and temperature, the best fit with experimental data was obtained by using a value of 1.5 x 10(-9) m(2)/s, which agrees well with available experimental data for strong NaCl-water electrolytes.