Two theoretical models for fluid flow and mass transfer at the trailing edge of a gas slug for small and large Reynolds numbers are suggested. In the case of small Reynolds numbers the creeping fluid flow at the trailing edge of a slug near a corner formed by a plane rigid wall and gas-liquid interface is investigated. The flow is caused by in-plane motion and by a fluid in the gap between a rigid wall and a gas-liquid interface. Using this model the rate of mass transfer from the bottom of a slug during gas absorption is determined. In the case of high Reynolds numbers the vortex flow at the trailing edge of the gas slug is investigated. A model of a fluid flow and mass transfer in a vortex flow in cavities is applied for the investigation of vortex formation at the trailing edge of a gas slug.