We examine the historical reanalysis of the Spanish sequence es que (lit. ‘is that’) as a discourse particle with justificatory and even purely emphatic functions. We argue that the diachronic process involved first the appearance of non-coindexed pro as subject of the copula es ‘is’ and, at a later stage, deletion of the empty pronoun and syntactic restructuring. The restructuring was triggered by the use of the construction in counter-argumentative and other contexts where the semantic content of the null category was not easily recoverable. We also consider parallel developments in Catalan and Portuguese, as well as the borrowing of the particle es que in colloquial Basque.