The present paper analyzes the discourse-pragmatic function of introducing Spanish qué ‘what’- interrogatives with the concessive connective pero ‘but’. In some contexts, a pero-preface contributes to the interpretation of the interrogative as the realization of an interactional challenge rather than a request for information (e.g. an information question). We explore the inferential processes by which the pero- preface leads to an interpretation of the interrogative as an interactional challenge and try to demonstrate that this challenge function of pero-prefaced qué-interrogatives may not only achieved ‘ad hoc’ by a local combination of the constitutive elements, but also by conventionalized form-function associations that developed diachronically. In a first step, we analyze pero-prefaced qué-interrogatives in a corpus of spoken Present Day Spanish. There are three main functions of pero-prefaces: to signal that a previous answer to the same interrogative is insufficient, to insist on an answer to a previously unattended request, or to challenge an immediately preceding action by an interlocutor. Using methodology from variationist linguistics, we identify entrenched patterns of pero-prefaced qué-interrogatives that have conventionalized the challenge function. In a second step, we conduct a diachronic variationist analysis of the development of Spanish pero-prefaced qué-interrogatives between 1700 and 1975, testing the hypothesis that the challenge reading developed later than the question reading. Our results show that due to their largely monological nature, the same inferential processes cued by pero lead to different discourse functions in historical texts. Over time, however, the use of pero-prefaced interrogatives started to become more likely in constructed dialogues. We argue that this change reflects an ongoing conventionalization of the challenge function in pero-prefaced interrogatives in spoken language.