The properties of an electric shock tube fitted with a diaphragm are examined. The diaphragm opening process and its effect on the motion of the shock wave are studied. A simple model to account for the diaphragm opening time is given and critical comparisons of theory and results with other work are made.The model works well both for shock tubes employing room temperature driver gas and also for those using heated driver gas. Furthermore, there is strong evidence that the diaphragm opening process is responsible for the accelerating phase of the shock wave motion in both types of shock tube.