Reasoning is a fundamental process in design activity, and it provides a way to understand design behaviour. Theories and models of design propose reasoning that follows abductive-deductive patterns. At the micro-level, these patterns are untested. This study analyses verbal reasoning patterns at the micro-level for group idea generation using protocol analyses of concurrent verbalisations from five design teams with industry participants. The results show that reasoning in design activity across 218 ideas follows general patterns of abductive-deductive reasoning. At the individual idea level, the reasoning patterns are disorderly and enter into micro-patterns of inference. The study concludes that understanding reasoning at early-stage idea generation processes is indicative of the mental models and abductive-deductive reasoning that are prevalent in design activity.