The present paper analyzes the mood alternation between Spanish conditional clauses introduced by the particle si ‘if’, on the one hand, and by conditional conjunctions such as a condición de que ‘on the condition that’, con tal de que ‘provided that’ and siempre y cuando ‘as long as’, on the other hand. Situated within the theoretical perspective of Cognitive Grammar, the paper argues that the conceptual content of the linguistic unit that introduces the conditional clause determines mood choice. In particular, it is claimed that the indicative mood in si-clauses reflects the conceptualizer’s reasoning about the causal relation between two events. By contrast, the occurrence of the subjunctive mood in other conditional clauses is analyzed as a natural effect of the condition expressed by the conditional conjunction. The analysis further shows that the mood alternation can be explained by the concept of dominion. The particle si introduces a clause that is located within the conceptualizer’s epistemic dominion, whereas other conditional conjunctions introduce clauses that are located outside the dominion of effective control.