Sedentary behavior (SB) poses significant health risks. Still, older adults frequently engage in SB, presumably driven by habit. As stable contexts and rewarding experiences reinforce habits, understanding these mechanisms is essential for designing effective interventions. This study used sensor-triggered ecological momentary assessments (EMA) in 90 older adults (age 70.2 +/- 6.4, 64.4% female) for fourteen consecutive days. Participants wore an ActivPAL accelerometer, a Fitbit activity tracker, and used a smartphone EMA app. Up to six times a day, they were prompted after 30 minutes of SB to report on their activity, context, habit strength, and reward value. Descriptive statistics explored habit strength, context stability, and reward value. Generalized linear mixed models examined the associations between these variables. Results showed that SB is both habit-driven yet also often rewarding, performed for pleasure, self-care, or personal values. Context stability depends on the sedentary activity. Activities performed in more stable social contexts and more rewarding activities were associated with higher habit strength. Interventions should adopt habit substitution strategies that disrupt the automatic response of sitting, while offering a substitute that is equally rewarding, in a different posture or with more frequent interruptions.