In situ thin film thickness monitoring and profilometering was implemented for a spray pyrolysis deposition technique, based on measuring the reflection of a laser beam from the film-substrate system; the interference pattern obtained during film deposition was used to calculate the film thickness. Recording in situ the reflection pattern for points separated 0.5 cm along the film allowed the film profile to be obtained. The growth of pyrolitic ZnO film on glass was studied by this method. It was found that the in situ measured thickness is in good agreement with that measured by ex situ profilometering or scanning electron microscopy (SEM). SEM micrographs of the films showed that the surface is formed by elongated-shaped particles, which become rounded and larger in size as the film becomes thicker. This fact was correlated with an increment of the diffuse component of the optical spectral reflectance and transmittance and with the trend of the surface roughness factor, obtained in situ. © 2005 IOP Publishing Ltd.