High penetration of social media in Indonesia gives more opportunities for sellers to promote their products, as well as for consumers to buy. Instagram is a popular social media for ecommerce in Indonesia. In 2016 Instagram introduced the Call-to-Action (CTA) buttons to trigger more e-commerce transactions on Instagram. On the other hand, impulse buying of social commerce consumers can be explained by the Stimulus-Organism-Response (S-O-R) model. Therefore, the objective of this study is to investigate the impact of the CTA buttons on consumers' impulse buying using the S-O-R model. A questionnaire was administered to collect data from 163 consumers. The data were analyzed and tested using Partial Least Squares (PLS). The results were validated by interviews with 3 sellers and 3 consumers. It shows that the CTA buttons have a positive impact on consumers' impulsive purchase, but less effective to improve consumers' perceived enjoyment. The main contribution of this study is the understanding of consumers' impulse buying response on promotional ads feature, which is related to the CTA buttons.