Abstract: A process of pressurization, of supply for effecting a dive for underwater intervention with a breathing mixture with hydrogen, and of decompression of a person making the dive, wherein the person is pressurized to an absolute pressure P.sub.1 of at least 0.45 MPa with a first type of breathing mixtures not containing hydrogen. The person is supplied at least from this pressure P.sub.1 with a second type of breathing mixtures at pressure P as a function of the depth of dive p to which said person is lowered, which second breathing mixture is of hydreliox type containing hydrogen at a minimum partial pressure of 0.33 MPa, oxygen at less than 4% by volume, helium with a partial pressure of more than 0.1 MPa and other gases such a nitrogen with a total partial pressure of less than 0.09 MPa. Supplying this second type of hydrogenated breathing mixtures in accordance with a composition which would locate the dive in one of the zones of the high pressure nervous syndrome or of narcosis is avoided.