To quantify the mediating role of leisure time physical activity (LTPA) and five dietary behaviours on educational differences in 13-year body mass index (BMI) gain across adulthood.Participants from the Melbourne Collaborative Cohort Study (4791 women; 3103 men) who maintained or gained BMI over 1990-1994 to 2003-2007 and met our inclusion criteria were selected. Education, potential mediators and confounders (age, alcohol, and smoking) were measured at baseline. We conducted sex-specific multiple mediation analyses using MacKinnon's product of coefficients method.A higher educational attainment was associated with a 0.27 kg m (95% CI 0.14, 0.39) lesser 13-year BMI gain among women only. We observed significant indirect effects of educational attainment on 13-year BMI gain through LTPA and nutrient-rich foods (each associated with a higher educational attainment and lesser 13-year BMI gain) and diet soft drink (associated with a lower educational attainment and greater 13-year BMI gain), which mediated 10, 15 and 20% of this relationship, respectively (45% in total).-2Nutrient-rich foods, LTPA and diet soft drink may represent effective public health targets to reduce inequities in excess weight across adulthood.