We tested the effect of soil moisture content on the efficiency of gas-phase ozonation for two types of soils containing residual petroleum. For the first soil (BM2), having a total petroleum hydrocarbons (TPH) concentration of 18,000 mg/kg soil, a moisture content of 5% benefited oxidation, giving the highest efficiency of ozonation for TPH removal and for producing soluble and biodegradable products. In contrast, higher moisture content hindered 03 from oxidizing reactive materials in the second soil (BM3), which had a higher TPH concentration, 33,000 mg/kg soil. This trend was documented by less TPH removal, less generation of soluble and biodegradable organic products, and a carbon balance that showed retarded carbon oxidation. An unexpected phenomenon was smoldering during ozonation of air-dried (<1% moisture) BM3, which did not occur with the same moisture conditions for BM2. BM3 smoldered was due to its higher TPH content, low heat buffering, and more release of volatiles with low self-ignition points. Smoldering did not occur for >= 5% water content, as it suppressed the temperature increase needed to volatilize the organics that initiated smoldering. The findings underscore the importance of controlling water content during ozonation to optimize the effectiveness of ozonation and prevent smoldering. (C) 2017 Elsevier B.V. All rights reserved.