Fragmentation and rapid conversion of forested landscapes to farmland and human settlements in Upper Assam, India has led to the isolation of western hoolock gibbon (Hoolock hoolock) families in fragmented patches of village areas. Many families have perished due to resource scarcity and conflict with development; however, the villages of Barekuri area in Upper Assam have retained a substantial number of gibbon families for many years. We monitored the population (nine families) from 2011 to 2017 and present long-term data on social organisation and its dynamics in Barekuri area. We recorded eight births, nine deaths and four dispersals in the population. Five of the nine deaths were due to electrocution that reduced the population size from 29 individuals in nine groups in 2011 (3.22 ± SD 0.67) to 24 in seven groups in 2017 (3.29 ± SD 0.76), with a mean group size of 3.67 ± SD 0.85 which did not differ over the years. Female inter-birth intervals and ages at first reproduction are comparable with those in wild populations. Both males and females took the opportunity to form groups and breed, and male replacement and female surrogacy indicate flexibility in a pair-bond. Our observations thus support a growing knowledge of variability in the social organisation of gibbons.