The study investigates upper secondary school students' use of counterfactual reasoning when engaging in a task concerning historical explanation. The study analyses student answers to a prompt asking them to evaluate the causal importance of a historical actor for a historical event, aiming to characterize the counterfactuals used, as well as applying possible criteria for what can be considered a qualified counterfactual. The criteria for qualification of counterfactuals are based on theoretical proposals about the potential of counterfactuals in relation to historical explanation. The findings indicate that a majority of the students involved use counterfactuals in their reasoning about explanatory importance, most of them employing counterfactual reasoning in relation to the historical actor. The analysis of qualification indicates that student reasoning becomes more qualified when students instead focus on structural factors, include both structures and actors in their counterfactual reasoning, or support their reasoning by making comparisons.