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 degrees. to less than 5 degrees. after treating at 120 to 180 degrees 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 degrees. 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.