There is a need to have reliable and accessible screening measures of mentalizing ability to investigate the role of this capacity in social functioning and its development, emergence of psychopathological conditions, as well as to understand its role in psychotherapeutic processes. The 8-itemed Reflective Functioning Questionnaire (RFQ-8) has been developed for this purpose, however its factor structure and psychometric properties are unclear. The aim of the current study was to further investigate the factor structure of the RFQ-8 and explore its psychometric properties in a Polish sample. 538 participants aged 17-50 (M = 21.44, SD = 2.92) took part in the study. They were asked to complete the RFQ-8, Difficulties in Emotion Regulation Scale (DERS), General Health Questionnaire (GHQ-28), the Relationship Structures (ECR-RS) questionnaire and the Borderline Personality Inventory (BPI). Results of both exploratory and confirmatory analysis indicated that the RFQ-8 may be regarded as a scale measuring a unidimensional construct. Our results confirmed good construct validity for the unidimensional RFQ-8, suggesting that the one-dimensional RFQ specifically assesses uncertainty concerning mental states. Our findings highlight the need for further studies investigating the factor structure and psychometric properties of the RFQ-8.