In this paper a multicast application is presented, based on active networking techniques, in which processing of the multicasted data is done in intermediate network nodes, according to requests or demands done by users joining the multicast session. The problem of transcoding in multicast sessions is divided into two subproblems. First some issues about the application of active networking techniques are briefly discussed. Then, the problem concerning the optimisation of the location of transcoding nodes in the multicast tree will be the main subject of this paper. The exact solution and a number of heuristics are discussed and simulations of the solutions will illustrate the advantages of the application of transcoding in multicast sessions.