Adult illiteracy remains a worldwide phenomenon in spite of numerous efforts being made by the government, church, community-based and non-governmental organizations. The phenomena surrounding adult illiteracy is fundamentally an abuse of human rights since, it deprives the adult their psychological, social and physical provisions. It is against this background that research was conducted in the Nairobi government prisons (GK prisons) to establish the extent to which physical learning environment influences the participation of inmates in the post-literacy adult classes. Survey and naturalistic designs were employed. Probability sampling procedures were employed to select three adult education officers, six prison welfare officers, 32 teachers and 96 learners (prisoners) and one adult education teacher. To obtain the information from the respondent, questionnaires, interview guide, observation schedule and document analysis were used. Data was collected and summarized into frequencies, percentages, graphs, cross tabulations, charts and narrative reports. The null hypotheses were tested using independent samples t-test and One-way ANOVA. The findings of the study revealed that effective participation in post-literacy project in prisons has been hindered by lack of qualified adult education teachers, lack of facilities and teaching and learning materials.