This article examines several research questions to establish a theory model for explaining factors that influence adult learners' preferences for constructivist Internet-based learning environments (CILE). Data were gathered from 541 individual participants enrolled in adult education institutes in Taiwan for structural equation modelling (SEM) analyses. The research results established a 10-scale instrument for assessing adult learners' preferences for CILE. Among the scales of CILE, adult students placed the highest value on relevance to life and reflective thinking, and scored the lowest for critical judgement, ease of use and student negotiation. The SEM analyses revealed that compared to Internet self-efficacy (ISE), self-directed learning readiness plays a major role in predicting adults' preferences for their learning environment, especially for the higher level of intellectual challenge. ISE plays a mediating role in the relationships between Internet usage and the participants' preferences. This result suggests that more time spent on Internet practice may increase adult learners' ISE, which may strengthen their preferences for Internet-based learning environments.