Purpose <p>There is growing interest in ecological momentary interventions to promote and improve mental health in youth. However, the lack of validated measurement tools for ecological momentary assessment may hinder implementation and evaluation of such interventions. This study aimed to provide an initial validation of the English version of the Ecological Momentary Well-being Instrument (EMoWI), which was designed for momentary measurement of adolescent well-being.</p> Methods <p>Both Classical Test Theory and Item Response Theory were used to assess dimensionality, reliability, and validity of the scale, as part of a cross-sectional approach, in a sample of 2,504 Canadian English-speaking adolescents from the COMPASS study. Examination of differential item functioning (DIF) was conducted on both gender and nationality, by combining the current sample with the sample of French adolescents used in the French version validation study.</p> Results <p>Confirmatory factor analysis supported a single factor hypothesis. Internal consistency was high and item fit statistics were satisfactory. Correlations with existing scales were consistent with pre-set hypotheses. We observed a significant overlap between respondents’ abilities and item difficulties, suggesting the EMoWI items adequately measure people’s level of well-being. DIF was statistically significant, yet negligible in effect size, for several items, thus allowing the use of score comparisons across genders and countries.</p> Conclusion <p>The EMoWI seems to be a promising candidate to measure momentary well-being in English-speaking adolescents. Subject to further psychometric validation in a momentary context, its use could inform the design and evaluation of ecological momentary interventions in the field of youth mental health in international studies.</p>