We propose a process algebra obtained by extending a combination of the process algebra with continuous relative timing from Baeten and Middelburg (Process Algebra with Timing, Springer, Berlin, 2002, Chapter 4), and the process algebra with propositional signals from Baeten and Bergstra (Theoret. Comput. Sci. 177 (1977) 381-405). The proposed process algebra makes it possible to deal with the behaviour of hybrid systems, i.e. systems in which the instantaneous state transitions caused by performing actions are alternated with continuous state evolutions. This process algebra has, in addition to equational axioms, rules to derive equations with the help of real analysis. (J.A. Bergstra), keesm@win.tue.nl (C.A. Middelburg) URL: http://www.win.tue.nl/∼keesm (C.A. Middelburg).0304-3975/$ -see front matter