The influence of an osmotic step and the syrup composition on the chemical-physical properties, drying kinetics, structure collapse and colour changes of air dehydrated tomato slices, was studied. Sugar solutions, with reduced sweetening capacity with or without CaCl2 added, were used. During air dehydration, colour attributes and geometric features were evaluated by image analysis, and an electronic nose was used to monitor the changes in aroma profile. The osmotic pre-treatment increased drying rate during the first falling rate phase of the air dehydration step but in the last phase reduced it. Sugar enrichment improved colour stability during subsequent air dehydration and helped to slightly decrease shrinkage during the first phase. The calcium addition improved surface area retention. Image analysis has proved to be a useful tool to study structure evolution during dehydration in combination with conventional analysis. The electronic nose was able to characterize the process aromatic fingerprint, helpful in understanding and parameterizing the degradative events during dehydration. (c) 2007 Elsevier Ltd. All rights reserved.