Purpose: This paper aims to present the results from a questionnaire distributed to a group of Spanish postgraduate teacher students pursuing a Master's Degree in Secondary School Education. The aims of the administered questionnaire were to identify students' perceptions of the development of sustainability competencies; to analyse the relationship between students' perceived level of competence and the subject area, previous teaching experience and previous participation in sustainability projects; and to explore the strengths and limitations of the assessment tool used. Design/methodology/approach: An "ad hoc" questionnaire of 18 items was designed to analyse students' self-conception of the development of four sustainability competencies. A set of six units of competence were identified and three levels of acquisition for each unit Knowledge (referred to conceptual learning), Knowhow (related to practical skills) and Do (linked to the demonstration in action and its transferability to real-life situations). The sample included 183 postgraduate secondary teacher students from different disciplines and subject areas. Findings: The findings of this study show that students positioned themselves for the four sustainability competencies in a medium level of competence. No statistical significant differences exist between the subject areas and the level of sustainability competence. A statistically significant difference was found between previous teaching experience and participation in sustainability projects in relation to their perceived level of sustainability competencies. Participation in sustainability projects is clearly shown as a differentiating factor in the levels of sustainability competencies. Originality/value: An empirical study has been conducted to investigate preservice teachers' perceptions on the self-development of four sustainability competencies, considering three levels of acquisition (Know, Knowhow and Do). This study provides insights into ESD teaching and learning approaches and the assessment of Education for Sustainable Development outcomes. It also points out the importance of conceptualising sustainability competencies and operationalising these competencies in assessment tools that can help measure sustainability competencies' development.