Thin nano-granular ZnO layers were prepared using a sol-gel synthesis and spin-coating deposition process with a thickness ranging between 20 and 120 nm. The complex dielectric function (epsilon) of the ZnO film was determined from spectroscopic ellipsometry measurements. Up to a critical thickness close to 60 nm, the magnitude of both the real and the imaginary parts of epsilon rapidly increases and then slowly tends to values closer to the bulk ZnO material. This trend suggests a drastic change in the film porosity at both sides of this critical thickness, due to the pre-heating and post-crystallization processes, as confirmed by additional characterization of the structure and the morphology of the ZnO films. (C) 2015 Elsevier B.V. All rights reserved.