In waterflooded reservoirs, it is possible to recover a significant amount of residual oil by enhanced oil recovery. Immiscible water-alternating-gas (WAG) injection is one of the well-establisbed methods for improving oil recovery. However, the mechanism of three-phase flow in the process has not been well understood and prediction of the three-phase permeability has been highly uncertain. This paper presents the results of immiscible WAG injection in a water-wet micromodel. During immiscible gas injection after an initial waterflood, gas moved through the residual oil paths, and residual oil was pushed either toward the production end of the model or into previously waterflooded channels. Breakthrough of gas occurred at about 0.25 PV for the micromodel used in this work. Further gas injection beyond the breakthough volume increased oil recovery only very slightly. When water was injected following gas injection, it flowed through channels that were created in the initial waterflood. Most of the residual oil that had been pushed into these waterflooded channels by the previous gas injection was produced. The mechanism of gas, oil, and water flow during immiscible WAG injection was analyzed. The observations and analysis provide insight into the flow behaviour of a three-phase system in the immiscible WAG process, which is important in the modelling of the process.