CSP processes have a static view of their environment – a fixed set of events through which they synchronise with each other. In contrast, the π-calculus is based on the dynamic construction of events (channels) and their distribution over pre-existing channels. In this way, process networks can be constructed dynamically with processes acquiring new connectivity. For the construction of complex systems, such as Internet trading and the modeling of living organisms, such capabilities have an obvious attraction. The occam-π multiprocessing language is built upon classical occam, whose design and semantics are founded on CSP. To address the dynamics of complex systems, occam-π extensions enable the movement of channels (and multiway synchronisation barriers) through channels, with constraints in line with previous occam discipline for safe and efficient programming. This paper reconciles these extensions by building a formal (operational) semantics for mobile channels entirely within CSP. These semantics provide two benefits: formal analysis of occam-π systems using mobile channels and formal specification of implementation mechanisms for mobiles used by the occam-π compiler and run-time kernel.