The chemical bath deposition of Zn(Se,O) thin films is studied with a quartz crystal microbalance (QCM). The deposition velocity is measured with QCM under different experimental conditions, including substrate properties, bath temperature and bath composition, The plots of the growth velocity vs. time reflect the sequence of processes for the deposition of the film. At the beginning, an induction period takes place, which ends up with the fort-nation of first ZnSe particles. These particles activate an electroless reaction induced by a reducer, hydrazine or hydroxide, which gives rise to deposition of a film with ZnO and ZnSe composition, This step is necessary to attain compact and thicker films with appropriate conditions for solar cell application, The electroless mechanism is decelerated after deposition of some nanometers of film, hence, a second growth mechanism starts to predominate consisting in the chemical reaction of soluble selenide with hydroxide and Zn2+ cations on the substrate surface. The layers resulting from this second mechanism have higher proportion of ZnSe than the previously electroless generated layers. At longer times, the predominant growth mechanism is the deposition of ZnSe clusters formed in the solution, which produces less compact and poorly adherent top layers. This sequence of mechanisms gives rise to films with heterogeneous morphology and composition in depth.