  This volume contains the proceedings of the Combined 20th International
Workshop on Expressiveness in Concurrency and the 10th Workshop on Structural
Operational Semantics (EXPRESS/SOS 2013) which was held on 26th August, 2013 in
Buenos Aires, Argentina, as an affiliated workshop of CONCUR 2013, the 24th
International Conference on Concurrency Theory.
  The EXPRESS workshops aim at bringing together researchers interested in the
expressiveness of various formal systems and semantic notions, particularly in
the field of concurrency. Their focus has traditionally been on the comparison
between programming concepts (such as concurrent, functional, imperative, logic
and object-oriented programming) and between mathematical models of computation
(such as process algebras, Petri nets, event structures, modal logics, and
rewrite systems) on the basis of their relative expressive power. The EXPRESS
workshop series has run successfully since 1994 and over the years this focus
has become broadly construed.
  The SOS workshops aim at being a forum for researchers, students and
practitioners interested in new developments, and directions for future
investigation, in the field of structural operational semantics. One of the
specific goals of the SOS workshop series is to establish synergies between the
concurrency and programming language communities working on the theory and
practice of SOS. Reports on applications of SOS to other fields are also most
welcome, including: modelling and analysis of biological systems, security of
computer systems programming, modelling and analysis of embedded systems,
specification of middle-ware and coordination languages, programming language
semantics and implementation, static analysis software and hardware
verification, and semantics for domain-specific languages and model-based
engineering.
