The diffraction of a normally incident plane wave by a grating consisting of thin semi infinite chiral slabs is considered. The chiral slabs are simulated by the appropriate transition boundary conditions. The problem is simplified by decoupling $E_z$ and $H_z$ components via a similarity transformation. Then the problem is reduced to the scalar Riemann–Hilbert problems and solved in an explicit form. The expansion of the diffracted field in terms of the plane waves is obtained and the numerical results are discussed.