Using nonlinear ideal MHD equations, we analyse the structure of the convection region within a Petschek-type model for reconnection. We show how, assuming that the normal field and flow components remain small and using simple wave analysis, the structure of the reconnection layer as well as the behaviour of the tangential field and plasma parameters can be specified in terms of the external parameters in the inflow regions. Equations for the normal field and flow components and the angular width of the reconnection layer (assuming planar symmetry and a steady state) are also given in terms of the value of the electric field along the reconnection line. The model is suited to application at the earth's magnetopause and in the distant magnetotail. In particular, it lends itself to an investigation of the requirements for reconnection to occur and the general validity of MHD reconnection models in practical applications.