This study explores the unique role of Ga in amorphous (a-) In–Ga--O oxide semiconductors through combined theory and experiment. It reveals substitutional effects that have not previously been attributed to Ga, and that are investigated by examining how Ga influences structure—property relationships in a series of pulsed laser deposited a-In–Ga--O thin films. Element-specific structural studies (X-ray absorption and anomalous scattering) show good agreement with the results of ab initio molecular dynamics simulations. This structural knowledge is used to understand the results of air-annealing and Hall effect electrical measurements. The crystallization temperature of a-IO is shown to increase by as much as 325 °C on substituting Ga for In. This increased thermal stability is understood on the basis of the large changes in local structure that Ga undergoes, as compared to In, during crystallization. Hall measurements reveal an initial sharp drop in both carrier concentration and mobility with increasing Ga incorporation, which moderates at > 20 at% Ga content. This decline in both the carrier concentration and mobility with increasing Ga is attributed to dilution of the charge-carrying In--O matrix and to increased structural disorder. The latter effect saturates at high at% Ga.