Trace elements like molybdenum (Mo) are necessary in human body for the function of some important enzymes e.g., aldehyde oxidase, xanthine oxidase, etc. The characteristic constituent of these two important enzymes is the presence of two Mo atoms in +5 oxidation state along with two flavins per enzyme molecule. Riboflavin (RF), vitamin B2, a primary component in flavoenzymes has isoalloxazine moiety which can chelate a metal like Mo. The fact that RF can act as an acceptor or a donor of electrons forms the basis for its ability to facilitate electron- transfer when incorporated into flavoproteins. To envisage the interaction between RF and Mo(+5) complex ion [M] in vitro, we set up a model where RF is made to interact with [M] in varying concentrations of aqueous perchloric acid at 300K. We investigated such interaction by absorption and emission spectroscopy, life time measurement and also some calculations of the related thermodynamic parameters. The existence of equilibrium during the ground state complex formed here is explained in terms of observations of isosbestic point and isoemissive point. Fluorescence quenching of the solution of the complex formed is observed with the increasing concentration of perchloric acid, the Stern-Volmer constant being measured. The dynamic nature of the fluorescence quenching is predicted from the life-time measurement of RF in presence and in absence of [M]. The structure of the complex in solution can be explained from the electron-transfer property of RF. The equilibrium constant and the enthalpy of formation for such interaction are also measured.