Thin layers with controlled Sn/Te ratio (0 divided by 2) and thickness are prepared by coevaporation of TeO2 and Sn. The structure and chemical composition of the as-deposited layers and the changes they undergo upon heating are studied by various transmission electron microscopy techniques and energy dispersive spectroscopic and electron spectroscopy for chemical analysis. It is shown that during the coevaporation a chemical reaction between TeO2 and Sn takes place resulting in the production of a finely dispersed phase of elemental Te. Upon annealing transport of the dispersed elemental Te to the free upper surface of the layer, particle growth and crystallization are observed. A strong correlation between the concentration of elemental Te in the layer and the rate of these processes is found. The reaction between TeO2 and Sn proceeds till the complete consumption of TeO2 or Sn; SnTe is formed only at excess amounts of Sn and after all TeO2 is consumed.