[en] MESODIF-II is a variable trajectory plume segment atmospheric transport model designed to predict normalized air concentrations and deposition of radioactive, but otherwise non-reactive, effluents released from one or two levels over the same position in an xy-plane. In such a model, calculated particle trajectories vary as synoptic scale wind varies. At all sampling times the particles are connected to form a segmented plume centerline; the lateral and vertical dimensions of the plume are determined by a parameterization of turbulence scale diffusion. The model development arose from the need to assess the radiological effects of routine operations of commercial nuclear power reactors. The purpose of the present document is to make MESODIF-II and its peripheral programs available to those who wish to use it directly. The theory and mathematics in the model are explained. A user's guide, which treats not only MESODIF-II descriptively, but also other programs that have to be used to generate the input wind and stability data is included. Two other peripheral programs that are used in conjunction with MESODIF-II are also described. One is Program Stagg, the other is Program RERITE. A complete test run of MESODIF-II is included, as is the complete code for the program and the control cards used on the CDC-6400. A glossary for the MESODIF-II code, and a reference list arealso provided