After diffusion temperature and time, the most important parameters in capsule diffusion source behavior are the dopant vapor pressure characteristics in the capsule. The vapor pressure behavior is a function of the degree of homogenization of the Si-dopant system and hence of the source preparation technology. Various methods of preparing Si-As source material are discussed. Homogenized-source preparation is described as it relates to reproducibility and controlled capsule diffusion behavior. The measurement of the kinetics of arsenic vapor pressure development in a typical capsule system and the role of vapor pressure in determining capsule diffusion behavior are described. Finally, the dependence of arsenic vapor pressure characteristics, and hence of the diffusion, on source weight-to-capsule volume ratio is described.