This article presents the main features in the design of vacuum systems for electron cyclotron resonance (ECR) ion sources. It is very interesting to consider them as a possible source of high-charge-state ions in upgrading present heavy-particle accelerators. Scaling relationships are given as a basis for understanding the vacuum configurations of ECR ion sources. The relevance of gas desorption effects to vacuum equilibrium and stability is also pointed out. Ion-induced pressure instability, a model of hydrogen diffusion, and the influence of neutral gases on the vacuum stability of ECR ion sources are discussed.