Previous research has shown that check-all-that-apply (CATA) and forced-choice (FC) question formats do not produce comparable results. The cognitive processes underlying respondents' answers to both types of formats still require clarification. This study contributes to filling this gap by using eye-tracking data. Both formats are compared by analyzing attention processes and the cognitive effort respondents spend while answering one factual and one opinion question, respectively. No difference in cognitive effort spent on the factual question was found, whereas for the opinion question, respondents invested more cognitive effort in the FC than in the CATA condition. The findings indicate that higher endorsement in FC questions cannot only be explained by question format. Other possible causes are discussed.