Evaluating volcanic hazards requires knowledge of the processes that trigger eruptions and the nature and timing of geophysical signals related to these processes. One approach to addressing this need is to link geophysically observable signals (e.g., those related to seismic, aeromagnetic, inflationary, or degassing processes) to pre-eruptive magmatic events deduced (in hindsight) from studies of erupted magmas. Here we present data on sanidine crystals from the A.D. 79 eruption of Vesuvius that show abrupt changes in Ba concentration caused by magma chamber recharge events prior to eruption. These changes have been degraded by diffusion during the time interval between recharge and eruption, and we have determined the length of this time interval by modeling the measured Ba concentration gradients. The results identify three distinct recharge events in the century before the eruption, the most recent occurring ∼20 yr beforehand.