The implementation and results of World-Wide Web experiments on belief bias in syllogistic reasoning are reported. Closely parallel data were obtained both in traditional laboratory experiments and on the Web, providing strong support for the validity of World-Wide Web research. Owing to the large sample sizes, the Web experiments helped to estimate solution percentages and model parameters with greatly enhanced accuracy as compared to the laboratory and made it possible to precisely assess the relative merits of competing accounts of belief bias. It is concluded that the Internet is a useful and promising new research tool for the study of human deductive reasoning.