Purpose – The purpose of this paper is to investigate the effect of perceived product similarity and comparative ad claims on brand responses. Design/methodology/approach – This study uses a two (similarity between the target product and the comparison product: relatively similar vs dissimilar) by three (product attributes of the target product: common to the comparison product, distinct from the comparison product, and a combination of common and distinct attributes) between-subjects factorial design with 300 Thai undergraduate students. Findings – It finds that when perceived similarity between the products is high, a combination of superiority (distinct) and parity (common) ad claims lead to the best brand responses. When perceived similarity is low, superiority claims bring about the best brand responses. Research limitations/implications – It extends comparative advertising and category-substitution research by addressing the research gaps in perceived similarity and claim type. Practical implications – Companies should emphasize a product’s superior attributes in general but a combination of common and superior attributes when the product is relatively similar to other products in comparative advertising. Originality/value – This study provides new evidence that perceived product similarity moderates the effect of comparative ad claims on brand responses.