BACKGROUND: When losing weight, most individuals find it difficult to maintain a healthy diet. Social environmental conditions are of pivotal importance in determining dietary behavior. To prevent individuals from lapsing, insight in social environmental predictors of lapse in dietary behavior is needed. PURPOSE: Identify social environmental predictors of lapse in dietary behavior, using ecological momentary assessment (EMA) amongst Dutch adults trying to lose weight. METHODS: Adults (N = 81) participated in two 7-day EMA weeks. Six times a day semi-random prompts were sent. At each prompt, participants indicated whether a lapse had occurred and responded to questions assessing social support, descriptive norm, injunctive norm, social pressure, presence of others, and current location. Generalized estimating equations were used to examine associations with lapse. RESULTS: Injunctive norm (OR = 1.07, 95% CI = 1.03-1.11), descriptive norm (OR = 1.04, 95% CI = 1.02-1.07), and social pressure (OR = 1.09, 95% CI = 1.05-1.14), all toward diverting from diet plans, predicted lapses. Social support toward sticking to diet plans and presence of others did not predict lapses. When controlling for a prior lapse, all other associations became nonsignificant. Lapses occurred most often at home and gradually occurred more often during the day. CONCLUSIONS: Traditional public health perspectives have mainly focused on individual choice and responsibility for overweight related unhealthy lifestyles. This study shows that there may be opportunities to enhance intervention programs by also focusing on social norms and social pressure. The involvement of partners or housemates may create more awareness of the impact of (unintentional) social pressure on risk of lapsing, and reduce the level of exerted social pressure.