This paper presents a sliding-mode control method for automotive Selective Catalytic Reduction (SCR) systems to regulate ammonia coverage ratios to desired levels. SCR systems have been widely adopted on Diesel powered vehicles to reduce tailpipe NOx emissions. To maintain a high NOx removal efficiency as well as acceptable ammonia slip, the SCR ammonia coverage ratio has to be critically controlled to balance such a tradeoff. A sliding-mode control method is analytically developed to track the target ammonia coverage ratio. Its tracking performance under bounded measurement noise is evaluated. The control input saturation is explicitly considered and its effect on SCR transient behavior is assessed in the design procedure. Simulation results indicate the proposed control can effectively track the target ammonia coverage ratio and comparative study against a traditional PID control demonstrates the improvement of robustness in the presence of bounded disturbances.