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Surface charge decay on thick flat samples of HTV silicone rubbers charged by impulse corona is studied. In the experiments, surfaces of the materials exposed to corona were kept open to ambient air whereas the opposite surfaces were in contact with a grounded copper plate and surface potential distributions on the samples were measured using Kelvin type electrostatic probe. The developed procedure allowed for implementation of three study cases when (i) neutralization of pre-deposited charges by free ions present in air was prevented and surface potential decay occurred mainly due to bulk neutralization; (ii) gas neutralization took place under natural conditions and (iii) gas neutralization was enhanced due to increased amount of free ions in ambient air provided by nearby corona. Potential decay observed only due to bulk neutralization was used to evaluate voltage dependent conductivity of the materials and allowed for comparing them with those measured by the standard method. Comparison of decay characteristics observed for different test conditions were used to evaluate the relative importance of each mechanism on the total process of charge decay.