BACKGROUND: Based on 31 concept-elicitation interviews of former smokers and individuals who wanted to lose weight, a questionnaire was developed simultaneously in three languages (French, US-English and US-Spanish). Draft items were generated in each language and culturally adapted into Spain-Spanish and UK-English. Face validity was assessed through 30 cognitive debriefing interviews; resulting in a 27-item questionnaire. OBJECTIVE: To validate a questionnaire that investigates the positive well-being associated with behavioral changes in adults. METHODS: The psychometric properties of the questionnaire were examined in US-English, US-Spanish and French through a stand-alone study in former smokers and individuals who wanted to lose weight. RESULTS: Based on an initial Principal Component Analysis (PCA) and item-item correlation analysis, 18 items were retained. A final PCA indicated that the questionnaire encompassed five dimensions: Serenity; Healthy Lifestyle, Support from Others, Self-Confidence, and Social Life. The questionnaire surpassed the threshold for internal consistency reliability for all dimensions and globally (Cronbach's alpha>0.8) in both behavioral changes and in all languages. All items surpassed the criterion for item-convergent validity and item-discriminant validity was satisfactory for 17/18 items. Correlations between the questionnaire and the Psychological General Well-Being index (PGWBi) scores confirmed its validity and indicated that it supplements information obtained through the PGWBi. Known-groups validity was satisfactory in individuals who wanted to lose weight based on the amount of weight lost, BMI reduction and number of weight loss attempts, but was inconclusive in former smokers because the abstinence status and the discontinuation of smoking cessation aids were not confirmed. CONCLUSION: Results support the reliability and validity of this questionnaire making it a useful tool for determining the positive well-being associated with behavioral changes. The questionnaire is currently undergoing additional validity testing in a population where smoking status is assessed.