In this study, the composition of electrosynthesized cobalt selenide thin films was analyzed by electrochemical quartz crystal microgravimetry (EQCM) in a continuous flow mode. First, cobalt selenide films were electrodeposited on a Pt-EQCM electrode using a 0.1 M Na₂SO₂ electrolyte solution containing Co and Se precursor species at different potentials. A combination of sequential stripping and EQCM frequency monitoring protocol enabled mapping of the free Co, free Se, and CoSe content of the electrodeposited films. It was found that the composition of cobalt selenide film was very sensitive to the electrodeposition potential. The targeted CoSe content was maximum at −0.7 V and the free Co content was minimum at this potential. On the other hand, the free Se content decreased from ∼45% at −0.6 V to nearly 0% at −0.9 V. The film composition as established by the continuous flow analysis mode stood in stark contrast with the static (batch) mode where no free Co was observed in the films. Factors in these compositional differences as well as in the EQCM electrode geometry (face-down vs. face-up) are finally discussed.