We introduce a stochastic extension of CCS endowed with structural operational semantics expressed in terms of measure theory. The set of processes is organised as a measurable space by the sigma-algebra generated by structural congruence. The structural operational semantics associates to each process a set of measures over the space of processes. The measures encode the rates of the transitions from a process (state of a system) to a measurable set of processes. We prove that the stochastic bisimilarity is a congruence, which extends the structural congruence. In addition to an elegant operational semantics, our calculus provides a canonic way to define metrics on processes that measure how similar two processes are in terms of behaviour. This paper treats a similar subject looking into the algebraic foundations of stochastic process algebras that are nowadays used to model complex real systems. This paper is a revised version of [12]. At the time of its first publication, this paper opened a new research direction revealing the necessity to involve measure theory into the research on structural operational semantics.Process algebras (PAs) [7] are formalisms designed to describe the evolution of concurrent communicating systems. For capturing observable behaviors, PAs are conceptualised along two orthogonal axes. From an algebraic point of view, they are endowed with construction principles in the form of algebraic operations that allow composing larger processes from more basic ones; a process is identified by its algebraic term. On the other hand, there exists a notion of nondeterministic evolution, described by a coalgebraic structure, in the form of a transition system. The algebraic and coalgebraic structures are not independent: Structural Operational Semantics (SOS) defines the behavior of a process inductively on its syntactic structure. In this way, classic PAs are supported by an easy and appealing underlying theory that guarantees their success.In the past decades probabilistic and stochastic behaviors have also become of central interest due to the applications in performance evaluation and computational systems biology. Stochastic process algebras such as TIPP [23], PEPA [26,27], EMPA [8] and stochastic π-calculus [37] have been defined as extensions of classic PAs, by considering more complex coalgebraic structures. The label of a stochastic transition contains, in addition to the name of the action, the rate of an exponentially distributed random variable that characterizes the duration of the transition. Consequently, SOS associates a non-negative rate value to each tuple state, action, state . This additional information imposes important modifications in the SOS format, such as the multi-transition system approach of PEPA or the proved SOS approach of stochastic π-calculus, mainly because the nondeterminism is replaced by the race policy.With the intention of developing a stochastic process calculus for applications in systems biology, in this paper we propose a stochastic version of CCS [34] ...