A thermal model has been developed to evaluate, from a thermal and performance standpoint, a preliminary design of a Neutral Buoyancy Cryogenic fluid delivery System (NBCS). The NBCS is an important component of a Neutral Buoyancy Portable Life Support System (NBPLSS) to be used by suited astronauts in underwater training for extravehicular activity (EVA) at Johnson Space Center, Houston, Texas.The Systems Improved Numerical Differencing Analyzer-1985 Fluid Integrator (SINDA'85/FLUINT) program is used to model the NBCS. The modeling decisions are described, based on details of tank construction and material selection, and include the breakdown of the model components into nodes and conductors. The modeling of positional transients which result from moving internal components are presented, including the control of cycling artifacts. The convection and boiling considerations of three tank fluids (cryogenic fluid, 250 psi N2/O2 gas, and water) are presented. Two piston designs are assessed according to performance criteria, with the presentation of temperature and heat transfer rate profiles. Suggestions for subsequent analysis and design modifications are offered.