The two-volume set LNCS 15342 and 15343 constitutes the refereed proceedings of the 26th International Conference on Information Integration and Web Intelligence, iiWAS 2024, held in Bratislava, Slovak Republic, during December 2-4, 2024. The 27 revised full papers and 25 short papers are presented in these proceedings were carefully reviewed and selected from 92 submissions. The papers are organized in the following topical sections: Volume I: Web Intelligence, Language Models, and AI-based Question Answering; Blockchain, Peer Reviews, and Digital Transparency; Machine Learning in Healthcare, Climate Change, and Human Behavior; Sequence and Similarity Search Techniques; and Knowledge Graphs, Databases, and Ontologies. Volume II: Recommender Systems and Data Personalization; Digital Forensics, Journalism, and Echo Chambers; Human-Computer Interaction, Music Therapy, and User-Centric Systems; and Environmental and Geological Data Science.