The amount of substance evaporated from the surface of a viscous dielectric liquid during nanosecond barrier discharge has been measured for the first time using a method based on the creation of a gas microbubble in the volume of liquid and subsequent generation of discharge in the bubble. By measuring increase in the bubble radius during discharge, it is possible to determine the amount of substance evaporated from the microbubble walls. It is established that millijoule energy deposition in discharge leads to the evaporation of several nanomoles of substance, and this amount grows with the deposited electric energy.