The systematic development of complex systems usually relies on a stepwise refinement procedure from an abstract specification to a more concrete one that can finally be implemented. The use of refinement operators preserving system properties is clearly essential since it avoids properties to be re-investigated at each development step. In this paper, we formalize the notion of refinement for processes described as terms of the security process algebra (SPA). We consider several information flow security properties and provide sufficient conditions under which our refinement operators preserve such security properties. Finally, we study how refinements can be composed still preserving the security of the system.