Diabetes and gestational diabetes mellitus (GDM) are growing problems worldwide. Women with histories of GDM have poorer dietary and physical activity behaviours, which may contribute to their increased risk of developing diabetes. Women with histories of GDM are a growing population in need of diabetes prevention programs, but existing prevention programs are typically developed for an older population. This study aimed to explore the acceptability of a telephone- or a group-delivered diabetes prevention program for women with previous gestational diabetes and to compare the characteristics associated with program engagement. Postpartum women participated in a lifestyle modification program delivered by telephone (n=33) or group format (n=284). The intervention included one individual session, five fortnightly group sessions and two review phone-calls at six and nine months. The telephone program consisted of seven telephone sessions. Semi-structured interviews on barriers and enablers to program engagement (defined as completing ≥80% sessions) were conducted before (Group) and after (Group and Telephone) interventions. The Health Action Process Approach theory was used as the framework for inquiry. In the telephone-delivered program 82% participants met the engagement criteria compared with 38% for the group-delivered program. Engaged participants (Group) had significantly higher risk perception, outcome expectancy, and activity self-efficacy at baseline (P<0.05). Both engaged and non engaged women faced similar barriers to participation, but engaged women were more likely to be able to resolve these barriers. There was a greater decrease in body weight (-1.45±3.9 vs -0.26±3.5, P=0.024) and waist circumference (-3.56±5.1 vs -1.24±5.3, P=0.002) for engaged vs non-engaged participants following group program completion. Telephone delivery was associated with greater engagement in postpartum women. Engagement was associated with greater reduction in weight and waist circumference. Further studies are required to confirm the effectiveness of telephone-delivered program for diabetes prevention in postpartum women.