The hydrophilic states of stainless steel surface treated by the separate or combined use of gaseous ozone and heat were investigated by measuring the contact angle of a water droplet, the content of the surface carbon contaminants, and the adsorption isotherm of water vapor. Super-hydrophilic states of stainless steel plates were achieved only by the combined gaseous ozone/heat treatment. The contact angle of a water droplet on the plate decreased from 81° to less than 5° after treating at 120 to 180 °C under 2 vol% ambient gaseous ozone. It was found that the super-hydrophilization proceeded according to two different mechanisms. One is the decomposition of surface contaminants. There was a good correlation between the surface carbon content and the water contact angle over the range of 15 to 81°. The other is the once removal of surface hydroxyl groups followed by chemisorption of water on these dehydroxylated sites. As a result of the removal of contaminants and the rehydration, the total number of surface hydroxyl groups on stainless steel became larger than that on the original surface.