The influence of crosslinking a smectic-A polysiloxane on the Landau-Peierls instability is investigated by X-ray scattering techniques. It is observed that the half value width of reflections at small angles increases with increasing crosslinking density, indicating a decrease of layer correlation. In this system, therefore, the crosslinking leads to a destabilisation of the smectic-A phase contrary to a polymethacrylate system. This fundamental property is obviously influenced by the chemical constitution of the crosslinking agent.