To account for infants’ perceptual and cognitive development, the constructivist model proposes that learning a new object depends on the capability of processing simpler lower-level units, and then integrating these units into more complex higher-level units based on their relationships, such as regular co-occurrence. Here, we demonstrate that the process of associating visual and auditory attributes to build a new multisensory object representation is not only observed in the course of development, but also in the course of infants’ in-the-moment information processing. After a brief familiarization session of learning two pairs of novel audiovisual stimuli, 15-month-old infants showed two components in pupil dilations over time: A rapid dilation was observed when processing perceptually novel compared to familiar stimuli, and a slower dilation was observed when processing novel combinations of familiar stimuli. However, in 10-month-old infants, only the effect elicited by novel stimuli was observed. Our results therefore demonstrate that detecting perceptual novelty occurred earlier than detecting association novelty in infants’ information processing. These results support the view that infants perceive newly-learned objects by processing their constituent attributes and then integrating these components, as suggested by the constructivist model.