Diffuse is a finite difference computer program which may be used to calculate the one-dimensional diffusion and trapping of atoms in a host material under general initial, source, and boundary conditions. Diffuse was originally written in Fortran prior to 1980, and rewritten in 1983 in order to run on the Sandia Cray computers in use through the early 1990's. When the mainframe computers were retired, the Gas Transfer System (GM) Development Department ported the code to the PC platform, where it ran as a simple console application. All graphical output was lost during this port. GM code developers have completed an upgrade that provides a Windows 95/NT Diffuse application and restores all of the original graphical output. This upgrade is called Diffuse II version 1.0. This report serves both as a users manual for Diffuse II version 1.0 and as a general software development reference.