In order to get more insight in the contribution of the shell upon the microbubble oscillating behaviour, we recorded the dynamic response of individual microbubbles dependent on the acoustic pressure using high-speed imaging. We investigated threshold and hysteresis effects in soft-shelled phospholipid microbubbles. In contradiction to the predictions of current bubble models, we have shown that depending on size, microbubbles need an initial acoustic pressure, before they start oscillating. This threshold behaviour was also found in the hysteresis curves of microbubbles below 5 μm. The presence of an oscillation threshold may be used to improve imaging techniques.