The invention relates to a catalyst comprising a titanium dioxide carrier and a mixture of metal oxides comprising at least one oxide of a metal selected from the group consisting of vanadium and tungsten and silicon oxide, comprised in such an amount that silicon (Si) is present in the catalyst in an amount of at least 1.0 wt %, relative to the weight of the catalyst. The invention also relates to a process for the preparation of fumaronitrile and/or maleonitrile by ammoxidation of C4-straight chain hydrocarbons in the presence of a catalyst according to the invention.