Cross-linguistic research has shown that object which-questions are the hardest types of wh-questions for children to comprehend and are acquired late. The present study asks when Spanish Differential Object Marking (DOM), an early cue to object marking, is actively used to successfully comprehend object which-questions in Spanish-speaking children. We also investigate whether DOM is first used in object which-question with human objects versus animal objects. Fifty-five child learners of Argentinian Spanish (N = 28, age 5-6; N = 27, age 7-8) and nine monolingual adults participated in a picture matching task. The experimental stimuli are which-questions where DOM indicated an object interpretation (¿Aqué bombero[subscript object]está salpicando la doctora[subscript subject]?) or where absence of DOM made the structure ambiguous between a subject and an object question (¿Qué bombero[subscript subject/object]está salpicando la doctora[subscript subject/object]?). The results showed that DOM helped adults arrive at an object interpretation for object which-questions. When DOM was absent, adult responses did not differ from chance. Children did not use the morphological marking to interpret object which-questions, and absence of DOM determined a strong subject interpretation. In addition, a developmental effect indicates that at age 7 to 8, a subgroup of the children performed target-like. Spanish-speaking children used an agent-first bias to interpret ambiguous which-questions (i.e., without DOM). However, by age 7 to 8, children start using DOM as a reliable cue to override or prevent commitment to an agent-first interpretation.