Reasoning is at the core of design activity and thinking. Thus, understanding and explaining reasoning in design is fundamental to understand and support design practice. This paper investigates reasoning in design and its relationship to varying foci at the stage of idea generation and subsequent performance of ideas developed. Understanding reasoning in design and its relationship to the performance of ideas generated is important to understand design activity, which can be used to develop tools or methods that can improve the effectiveness of design teams. Protocol analyses were conducted to investigate idea generation sessions of two industry cases. Reasoning was found to appear in sequences of alternating reasoning types where the initiating reasoning type was decisive. The study found that abductive reasoning led to more radical ideas, whereas deductive reasoning led to ideas being for project requirements, but having a higher proportion being rejected as not valuable. The study sheds light on the conditions that promote these reasoning types. The study is one of the first of its kind and advances an understanding of reasoning in design by empirical means and suggests a relationship between reasoning and idea performance. Findings of the study further allows for a way to analyse and improve the performance of idea generation in design teams.