Many adult smokers are intermittent smokers (ITS) who do not smoke daily. Prior analyses have suggested that, compared with daily smokers (DS), ITS smoking was, on average, more linked to particular situations, such as alcohol consumption. However, such particular associations assessed in common across subjects may underestimate stimulus control over smoking, which may vary across persons, due to different conditioning histories. We quantify such idiographic stimulus control using separate multivariable logistic regressions for each subject to estimate how well the subject’s smoking could be predicted from a panel of situational characteristics, without requiring that other subjects respond to the same stimuli. Subjects were 212 ITS (smoking 4–27 days/month) and 194 DS (5–30 cigarettes daily). Using ecological momentary assessment, subjects monitored situational antecedents of smoking for 3 weeks, recording each cigarette in an electronic diary. Situational characteristics were assessed in a random subset of smoking occasions (n = 21,539), and contrasted with assessments of nonsmoking occasions (n = 26,930) obtained by beeping subjects at random. ITS showed significantly stronger stimulus control than DS across all context domains: mood, location, activity, social setting, consumption, smoking context, and time of day. Mood and smoking context showed the strongest influence on ITS smoking; food and alcohol consumption had the least influence. ITS smoking was under very strong stimulus control; significantly more so than DS, but DS smoking also showed considerable stimulus control. Stimulus control may be an important influence on maintaining smoking and making quitting difficult for all smokers, but especially among ITS.