Although there is growing interest in using mHealth approaches to provide just-in-time [JIT] intervention elements tailored to current emotional states in real time, critical evidentiary questions remain unanswered. This study was a 'proof of concept' evaluation to see if, in the context of a manualized stress management program, adding JIT intervention reminders via mobile technology would enhance outcomes more than the provision of random (untailored) reminders (both also relative to a measurement only active control condition). Individuals participated in a stress management course and carried a palmtop computer for 11 days (2 days acclimation and assessment, 1 week intervention, and 2 days post-treatment assessment). For the intervention week participants were assigned to one of three groups and either: (a) only completed self-report assessments multiple times each data (using ecological momentary assessment [EMA]; (b) completed EMA and received random reminders to use stress management skills; (c) completed EMA and received JIT tailored reminders to use stress management intervention elements when high stress or negative affect were reported. Individuals receiving JIT reminders reported stressful events less frequently, lower stress severity, less negative affect, lower levels of a stress biomarker (cortisol), less frequent eating, less alcohol consumption, less smoking and better sleep quality than the other two groups at post-treatment assessment. Tailored mHealth interventions designed in conjunction with ambulatory assessment protocols to provide JIT intervention elements can improve the efficacy of a standard stress intervention above and beyond any effects of simple reminders or over the intervention training. Implications for mHealth and JIT interventions are discussed.