We present a theory for predicting fluid ejection for microfluidic systems that utilize the squeeze-film effect as the dominant pressure generation mechanism. We derive the equations of motion for such systems taking into account both viscous and inertial effects. The dominant viscous term is due to the squeeze-film effect, in which a thin layer of incompressible viscous fluid is ‘squeezed’ between a moving piston and an orifice plate. We apply Reynolds lubrication theory and derive an analytical expression for the squeeze-film pressure distribution, taking into account both flow through the orifice and restricted flow around the piston. The dominant inertial effects are due to the acceleration of fluid between the piston and the orifice plate, through the orifice, and around the piston. We derive an analytical expression for an effective fluid mass that accounts for these effects. We use these expressions in the equations of motion and obtain a theoretical model for predicting the behaviour of squeeze-film dominated microfluidic fluid ejectors. We demonstrate the theory via application to a practical device.