Electrochemical amperometric transducers measure electric current to detect the concentration of reagents. This 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 (3D) sensor model with a fixed potential of -350 mV on the electrode surface and Fe2+ ions, which included 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). The dominant process in the cell volume is convection, with ion velocities of the order of 10-5 m/s. However, migration is the dominant process in the vicinity of the electrode, and it determines the time interval for reaching a steady state. The realistic 3D 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 the investigation of electrochemical sensor cells.