We introduce Affective Visual Dialog , an emotion explanation and reasoning task as a testbed for research on understanding constructed emotions in response to visually grounded conversations. The task involves three skills: (1) Dialog-based Question Answering (2) Dialog-based Emotion Prediction and (3) Affective explanation generation based on the dialog. Our key contribution is the collection of a large-scale dataset, dubbed AffectVisDial, consisting of 50K 10-turn visually grounded dialogs as well as concluding emotion attributions and dialog-informed textual emotion explanations, resulting in a total of 27,180 working hours. Notably, the dataset spans a broad range of visual stimuli, covering human heritage and contemporary life, with an average per-turn answer length of about 12 words — 5 times that of the VisDial dataset — and explanations exceeding 28 words on average. We explain our determining design decisions in collecting the dataset, data inclusion and exclusion criteria starting from over 100K dialogs for quality control, and introduce the questioner and answerer tasks that are associated with the participants in the conversation. We propose and demonstrate solid Affective Visual Dialog baselines adapted from state-of-the-art multimodal models. Remarkably, the responses generated by our models show promising emotional reasoning abilities in response to visually grounded conversations. The dataset and code will be publicly available