In this study, the researchers developed the Web-based Probability Explorer (WPE), a mathematical microworld and investigated the effectiveness of the microworld's constructivist learning environment in enhancing the learning of probability and improving students' attitudes toward mathematics. This study also determined the students' satisfaction with the microworld's features. The study was conducted with a quasi-experimental, non-random pre- and post-test static group research design. The samples of the study consisted of 52 students from two secondary schools in Kuching, Sarawak, Malaysia. The findings showed that students' learning of probability was enhanced after interacting with the WPE. Their attitudes toward mathematics were also slightly improved. The students were satisfied with the features of the WPE. (Contains 4 tables and 4 figures.)