In order to determine the role played by the hydrogen atoms in a-Si : H, short range order has been investigated using Exafs at the silicon K-edge. Emphasis has been put on topological disorder ; low temperature measurements allowed us to look at its variations. Concerning hydrogen, a non homogeneous distribution has been deduced and a 2-d or 1-d repartition consistent with both theoretical prediction and small angle neutron scattering data is proposed.