Philosophiae Doctor - PhD / The objective of this study was to determine if introducing acceptance and commitment therapy in the prevention of mother to child HIV transmission (PMTCT) program using weekly mobile phone messages would result in improved mental health status of HIV-positive, pregnant women in Nigeria. The study used a quantitative approach using a Solomon four-group (two intervention and two control groups) randomised design to evaluate the impact of an acceptance and commitment therapy program. The study population was 132 randomly selected (33 per site), HIV-positive pregnant women attending four randomly selected PMTCT centres in Nigeria. Two were Intervention and two were Control sites which functioned as Intervention and Control groups. The intervention groups were exposed to one session of acceptance and commitment therapy with weekly value-based health messages sent by mobile phone for three months during pregnancy. The control groups received only post-HIV test counselling.