Let π : X −→ Y be a good quotient of a smooth variety X by a reductive algebraic group G and 1 ≤ k ≤ dim (Y ) an integer. We prove that if, locally, any invariant horizontal differential k-form onX (resp. any regular differential k-form on Y ) is a Kähler differential form on Y then codim (Ysing) > k + 1. We also prove that the dualizing sheaf on Y is the sheaf of invariant horizontal dim (Y )-...