Prior studies have shown that articulating and sharing rationales in traditional small-group activities contribute to the maintenance of common ground, members' knowledge awareness, and contribution awareness. It is likely that the importance of articulating and sharing rationales will be increasingly acknowledged in online crowdsourcing because in such a context, large-scale participation is expected with participants often not knowing each other and being flexible about their participation status (e.g., participants may join after the activity has started and leave before it completes), and thus more grounding efforts/support are expected. To better understand the role of shared rationales in online crowdsourcing, three experiments were conducted investigating whether and how rationale awareness affects the ideation crowdsourcing task and idea-evaluation crowdsourcing task based on the findings about the rationale awareness effects in small-group idea-generation activities. The results suggest that one's awareness of previous workers' rationales in the current task can slightly improve the average quality of generated ideas in an iterative approach. In addition, one's evaluation of an idea could be positively or negatively affected by the idea's rationale depending on the quality of the rationales. The results also suggest that showing previous workers' rationales in the ideation task may not be an effective approach for improving the best quality of generated ideas.