This paper concerns the document multi-structuring issue. For various use objectives, many distinct structures may be defined simultaneously for the same original document. For example, a documentmay have a first structure for logical content organisation (logical structure), and a second structure to express a set of content formatting rules (physical structure). We have already proposed a generic model, called MSDM, for the multi-structured documents, in which several important features were established. In this paper, we address the encoding problem of this kind of documents. We present a new formalism, called MultiX, which allows encoding the multi-structured documents efficiently. This formalism is based on the MSDM model and uses XML syntax.