In this Communication, the electrical breakdown of a static bubble in a water-filled capillary generated in a dc electrical field is studied. We present experimental results which indicate that the liquid layer between capillary and bubble wall can have an important influence on the breakdown mechanism of the bubble. The breakdown electrical field (atmospheric pressure) without a liquid layer in a (vapor) bubble is 18 kVcm. When a liquid layer is present, the electrical breakdown of an air bubble is observed at electrical fields typically two times smaller. Local plasma formation is observed in this case possibly due to bubble deformation.