Resonance is a foundational organic chemistry concept, but it is consistently misunderstood by undergraduate students. The development of a concept inventory--a multiple-choice assessment where the incorrect answer choices stem from commonly held alternate conceptions--on the concept of resonance could help organic chemistry instructors quickly identify alternate conceptions held by their students and make interventions. While concept inventories have been developed in a variety of areas in chemistry, there is no existing resonance concept inventory despite the difficulty students seem to have with the concept. In this study, open-ended resonance items were designed and administered to first-semester undergraduate organic chemistry students to develop items for a pilot version of the Resonance Concept Inventory (RCI). Validity evidence based on content and response processes for the data obtained using the items was acquired through feedback from organic chemistry faculty and think-aloud interviews conducted with students. The most commonly occurring alternate conceptions on the concept of resonance were identified, and a 14-item pilot version of the RCI was developed.