Chee-Seng Tan0000-0002-1394-687XCheng Siew MayNikolett Eisenbeck0000-0003-0454-3734Soon-Aun TanIntan Hashimah Mohd HashimDavid F. Carreno2025-10-302025-10-302025-04-1710.1177/00332941251330553https://dspace-cris.utar.edu.my/handle/123456789/11644The present study was conducted to test and compare the predictive power of the Mature Happiness Scale-Revised (MHS-R) and the Peace of Mind Scale (PoMS) on well-being and their overall psychometric properties using a longitudinal design. A total of 330 undergraduate students and working adults answered an online survey comprising the two scales as well as the measurements of stress, life satisfaction, and well-being. Two weeks later, 165 participants completed the second survey package. Confirmatory factor analyses (CFAs) supported the originally proposed single-factor solution in both scales although the removal of one item from the PoMS was necessary. Both the 6-item PoMS and the 7-item MHS-R demonstrated moderate inverse relationships with stress and were good predictors of future well-being, even when accounting for life satisfaction and stress. However, the MHS-R scores could predict well-being beyond the influence of the PoMS scores, a relationship not observed in the reversed order. Moreover, further CFA supported that peace of mind and mature happiness are related but distinct constructs. Both PoMS and MHS-R are thus concluded as effective tools for measuring inner harmony that contribute to enhanced well-being. Nevertheless, mature happiness represents a unique aspect of well-being beyond the impact of peace of mind alone. © The Author(s) 2025.eninner harmonylife satisfactionlongitudinal studyMalaysiaMature happinesspeace of mindstresswell-beingjournal-article