The dielectric properties of vacuum impregnated (VI) and non-vacuum impregnated (NVI) apple. var. Granny Smith. were measured at 2.45 GHz and 25 degrees C using the open-ended coaxial probe method and a network analyzer (HP-85070B) connected to a computer with suitable software. The effect of moisture content, water activity and porosity on the dielectric constant and loss factor was evaluated. The results suggest a water activity value near 0.9 (0.6 water content, dry basis) from which dielectric loss mechanisms seems to be notably increased, related to the availability of mobile water. (c) 2005 Elsevier Ltd. All rights reserved.