In H2 S the oxidation state of S is -2. This can be determined using the following rules:

1) The oxidation state of hydrogen is +1.

2) The sum of the oxidation states of all atoms in a molecule is equal to the charge of the molecule.

3) The oxidation state of an atom in its elemental form is 0.

Since H2 S is a neutral molecule, the sum of the oxidation states of H and S must equal 0. Therefore, the oxidation state of S must be -2, since 2(+1) + (-2) = 0.