We present the results of a numerical investigation of the mixing within drops caused by surface tension variations. With microfluidic applications in mind, we simulate drops surrounded by a fluid of equal density and viscosity. We investigate both stationary coalescing drops and steadily flowing drops, and study the influence of drop size ratio, viscous effects, and surface tension variations. We measure the mixing efficiency using the variance of the concentration distribution and find that surface tension variations may result in faster mixing than geometric effects.