Bubble disruption is associated with the response of ultrasound contrast agents (UCAs) exposed to high acoustic pressures. This behavior is important for bubble detection techniques as well as flow quantitation and some proposed therapeutic applications. Previous work has measured acoustically the disruption threshold and postdisruption echo from populations of microbubbles. This suggests a model for UCA disruption whereby ultrasound breaks their shell, leaving free gas bubbles. Diffusion of gas causes the bubbles to shrink and, consequently, reduces the measured backscatter echo over time. In this work, similar bubbles containing three different gases were measured and their echo behavior with time compared with a simple simulation based on diffusion of gas out of the bubble. It was found that, in general, the simulations and experiments compared well at low disruption pressures. Incorporating bubble fragmentation in the simulation model brought its results closer to experiment.