In the process industries there are many cases where substantial economies could be made, if valve actuators were available with response times shorter than those presently available, with traditional spring-opposed-diaphragm units, the performance of these actuators being inhibited by built-in loading, a low working pressure and flow passages which are restricted in relation to the swept volume of the actuator. The paper presents an analysis of a pneumatic actuator system with response characteristics that are much improved compared with those available from conventional spring-opposed-diaphragm units. The equations are developed which characterise the behaviour of the pneumatic actuator, and comparisons of experimental (using a specially designed test rig) and simulated results show the accuracy of the model to be good. Linear approximate models of the pneumatic actuator system then form a basis for the application of two linear controller-design procedures. The design methods employed are the linear quadratic optimal control with extended cost functions and the Horowitz approach to sensitivity reduction. The resulting closed-loop designs are found to be robust and to achieve their required performance specifications under both emergency and normal actuator operations.