Experimental determination and predictive equations are used to evaluate the effect of vacuum impregnation with sucrose solutions on thermal properties of apple samples (Granny Smith). Vacuum impregnation (VI) was carried out by immersion of samples in the sucrose solution and applying 50 mbar in the tank for 10 min; afterwards, atmospheric pressure was restored while samples remain immersed for 20 min. Thermal diffusivity was measured in non-impregnated (NVI) and impregnated samples in a transient heat conduction method. Thermal diffusivity changes due to VI were relatively low, whereas conductivity changes, calculated from thermal diffusivity, density and specific heat, showed very important changes. The proposed predictive model allows us to estimate changes in thermal properties due to VI in terms of product porosity and pore distribution and conditions of VI process. (C) 2000 Elsevier Science Ltd. All rights reserved.