Hybrid transition systems in their full generality describe continuous behaviour by a set of equations in each mode an algebraic or di erential equation for each state variable in terms of inputs and other state variables. Each discrete transition may be taken according to a non-deterministic time constraint. In this paper we restrict this model to time-deterministic discrete transitions. Thus, every transition is guarded by a condition g and has a xed delay t. Di erent transitions may have di erent delays including zero, but progress is enforced after the delay. Using this restriction and a composition operator which uses union of mode sets we then prove certain compositionality properties. In particular, that the parallel composition of two subsystems produces a system whose semantics is de ned in terms of semantics of its constituents provided that it has a run. The restriction is well-justi ed in a large class of control applications where the complex mode-changing software is ...