Relaxation of hyperpolarized 129Xe in a deflating polymer bag

► Longitudinal relaxation of 129Xe in a polymer bag is dominated by wall effects. ► The wall relaxation rate depends linearly on the surface-to-volume ratio. ► Bulk gas-phase relaxation is dominated by intrinsic Xe-Xe relaxation. ► Relaxation contributions from magnetic field gradients and oxygen are small. ► Relaxation may limit experiments when slow gas delivery rates are employed.