Soil vapor extraction (SVE) combined with air injection provides an efficient way for the cleanup of vadose zone contaminated by volatile organic chemicals (VOCs). A successful design of an SVE system, however, relies on a good knowledge of the induced gas flow field in the vadose zone. Analytical solutions are available to help understand the gas flow field at steady state. However, most SVE systems must pass a transient period before reaching steady (or quasi-steady) state, and the length of the period should be system-specific. This paper presents an analytical solution for transient gas flow in a vadose zone with extraction and injection wells. The transient solution approaches the steady state solution as time increases. Calculations have shown that for a shallow well (screened in a depth of less than 10 m) in a vadose zone with an air permeability of 1 darcy (10-12 m2) or larger, the system reaches steady state in just several hours. Decreasing the air permeability or increasing the screen depth increases the time to reach steady state. In practical applications the transient solution may be relatively insignificant in an SVE design. However, the solution can be important in site characterization through pneumatic tests. A procedure is provided for applying the dimensionless solution in estimating air permeability and air-filled porosity. An example is also given to use the transient solution for verifying numerical codes.