Three polymers, polyethylene terephthalate, a polyimide, and polyamide-6, were subjected to surface partial discharges up to electrical breakdown under varying ambient humidity conditions. It was found that increasing humidity increases lifetime of the samples until a critical value of relative humidity is attained, after which the polymers＇ lifetime dramatically decreases. This critical value depends on the hydrophylicity of the polymer and is lower as the ability of the polymer to absorb water increases. The evolution of the electrical time to breakdown with the relative humidity is explained in terms of changes in surface conductivity following variations in the ambient humidity.