The mechanisms governing the modification of the optoelectronic properties of low carrier concentration indium tin oxide (ITO) during reactive laser annealing (ReLaA) are investigated. ReLA combines the advantages of reactive ambient thermal annealing and laser annealing; utilizing laser processing of room-temperature sputtered ITO thin films in pressurized reactive environments to probe the films’ crystal structure and defect composition. Advanced ellipsometric modelling (considering depth-inhomogeneity and intra- and inter-grain carrier transport), cross-sectional transmission electron microscopy, and X-ray photoelectron spectroscopy revealed ReLA-induced depth-dependent compositional and structural modifications that tuned the carrier concentration ((0.3 − 1.27) × 1020 cm−3) and epsilon-near-zero domain ( 3.07 − 11.7 μ m ) across a wider range than previously reported.