Electrochemical amperometric transducers measure electric current to detect the concentration of reagents. The present study examined the mass transfer processes in an electrochemical sensor operating in flow conditions using a numerical model. The transient physical phenomena were investigated in a realistic three dimensional sensor model with a fixed potential of -350mV on the electrode surface and Fe+2 ions, which includes diffusion, convection and migration mass transfer processes. Numerical simulations were used to identify the dominant physical processes for different conditions (inlet velocity and electrode surface potential). It was found that the dominant process in the cell volume is convection, with ions velocity of the order of 10-5 m/s. On the other hand, the migration is the dominant process in the vicinity of the electrode, and as a result determines the time interval for reaching steady state. The realistic model was compared with a simplified model that considered convection and diffusion only. Significant differences were found in the calculated normalized electric current which is equivalent to the ion concentration on the electrode, demonstrating the importance of incorporating migration in any investigation of electrochemical sensor cells.