This paper introduces a novel algebra for reasoning about step reactions in synchronous languages, such as macro steps in Harel, Pnueli and Shalev's Statecharts and instantaneous reactions in Berry's Esterel. The algebra describes step reactions in terms of configurations which can both be read in a standard operational as well as in a model-theoretic fashion. The latter arises by viewing configurations as propositional formulas, interpreted intuitionistically over finite linear Kripke structures. Previous work by the authors showed the adequacy of this approach by establishing compositionality and full-abstraction results for Statecharts and Esterel. The present paper generalizes this work in an algebraic setting and, as its main result, provides a sound and complete equational axiomatization of step reactions. This yields, for the first time in the literature, a complete axiomatization of Statecharts macro steps, which can also be applied, modulo encoding, to Esterel reactions.