New techniques are required to explore directly the kinetics of water transport in aerosol between the gas and condensed phases, both at high relative humidity (RH) close to saturation and at low RH where the role of amorphous states must be considered. Here, we present micro-Raman measurements of the kinetics of water transport between the bulk of a particle and the surrounding gas phase by examining the rate of exchange of D(2)O by H(2)O in droplets initially composed of MgSO(4)/D(2)O. The formation of an amorphous gel inhibits the response of the droplet composition to changes in the ambient RH and leads to a substantial reduction of the mass transfer rate of water in the droplet bulk with an apparent diffusion constant of 10(-15) to 10(-14) m(2) s(-1). These measurements are consistent with the imposition of a kinetic limitation on the time response of the aerosol particle size to changes in RH.