There is a growing literature providing evidence that not only cognitive skills but also non-cognitive skills are important for economic and social outcomes. This paper assesses the measurement properties of the Big Five and Grit scales in a large representative sample of adults in Poland. The data from the Polish Follow-up Study on the Programme for International Assessment of Adult Competencies (postPIAAC) include longitudinal information on PIAAC respondents in Poland and additional background information not available in the international study. The results presented in this paper show that not all the criteria concerning the reliability, validity and comparability of these scales have been satisfied, though the personality measures significantly contribute to explaining the variability in policy-relevant outcomes. Most of the questions discriminate well between people possessing a high and a low level of a given trait, though reverse-worded items perform weaker. The Big Five theoretical five-factor structure was not replicated; however, a six-factor model with an additional factor loading reverse-worded items fits the data. In case of Grit, a bi-factor model, which has an equivalent interpretation to the second-order theoretical structure, holds. The scales are not fully measurement invariant. The results confirm earlier findings from the literature that differences in personality traits are clearly associated with differences in life outcomes. For most of the outcomes, the Big Five traits outperform cognitive skills in predictive power. Only educational attainment is more strongly related to cognitive skills, while for wages, the predictive power of personality and cognitive skills is similar. The paper provides recommendations for incorporating measures of personality traits into competence surveys.