Electrochemical quartz crystal microbalance (EQCM) has been employed to study the potential oscillatory mechanism for the IO- 3 reduction accompanying periodic hydrogen evolution. The new experimental results that were obtained by in situ EQCM monitoring clearly demonstrate the all key steps involved in the oscillation: the diffusion-limited depletion of IO-3 by reduction, the formation, growth and departure of hydrogen bubbles on the surface, and the convection-induced replenishment of IO-3 by the hydrogen evolution. In addition to the frequency response to the surface mass change as reported in the literature, our study first shows that simultaneous frequency responses to the changes of density and viscosity in the diffusion layer during the oscillation can also provide meaningful and even decisive information on the oscillatory mechanism for the oscillators involving the coupling of electrochemical reactions with diffusion and convection mass transfer.