Comparison challenge approach is proposed as a form of challenger-activated. just-in-time Internet advertising. To develop a framework for a comparison challenge, we propose a theory of comparison value. A comparison is regarded valuable if a comparison opportunity is available and if the comparison is relevant and informative, has an appropriate level of detail, and is advantageous and trustworthy. Based on this theory, the CompareMe and CompareThem strategies are devised, and comparable objects are classified in terms of price and performance dominance as well as the scope of proximity. The idea is demonstrated with a comparison of PCs from five leading manufacturers. To assist in the planning of the comparison challenge, a mathematical programming model was formulated to maximize the value of comparison under the constraints of the comparison opportunity and budget. The model is applied to eight scenarios in terms of the range of comparing objects. The models under various scenarios are tested and contrasted with the real-world example of PCs. We found the ad effect of comparison challenge to be substantially better than banners (4.75 times) and similarity-based comparisons (2.77 times), providing customers with better performance and reduced prices.