Mobile health technologies are increasingly important for behavior change in chronic disease’s prevention and management. Yet, user diversity limits the efficiency of one-size-fits-all solutions. This study examines how age and gender influence preferences for 14 mechanisms for behavior change. In a cross-sectional online survey (N=214, M_age=29.4), logistic regressions tested these effects. Four mechanisms, self-monitoring, progression, challenge, and quests were universally preferred. Participants aged over 35 years disliked rewards (OR= 0.24, p<.01) but preferred prompts and cues (OR= 1.52, p=.05); women preferred avatars (OR= 2.39, p<.01) and were less attracted to social comparison (OR=-.83, p<.05) and competition (OR= 0.34, p<.01). These findings highlight the need for personalized mHealth designs to optimize engagement and effectiveness.