This book constitutes the proceedings of the 12th International Symposium on Frontiers of Combining Systems, FroCoS 2019, held in London, UK, in September 2019, colocated with the 28th International Conference on Automated Reasoning with Analytic Tableaux and Related Methods, TABLEAUX 2019.  The 20 papers presented were carefully reviewed and selected from 30 submissions. They present research on the development of techniques and methods for the combination and integration of formal systems, their modularization and analysis. The papers are organized in the following topical sections: automated theorem proving and model building, combinations of systems, constraint solving, description logics, interactive theorem proving, modal and epistemic logics, and rewriting and unification.