A detailed study of signal-charge injection into surface channel CCDa as a response to a gate-voltage step was carried out. The analysis is based on the time dependent diffusion equation with the fringing field term included. With the appropriate boundary conditions, this equation was applied to the input diffusion, input gate and succeeding gate of a CCD. It was found that injection of signal-charge into the device is characterized by three distinct processes, First is the transient-time required by the input diffusion to establish a constant injection-current under the input gate, then, the input stage enters into a constant current injection mode, and finally when the surface potentials of the input and receiving gates equalize, the injection current decays to zero. An analytical expression for the transient component, and for the total time required to inject a packet of charge into the CCD is derived.Experimental results carried out on a CCD shift register with various methods of charge injection, support the theoretical predictions. © 1976 Taylor and Francis Group, LLC.