Education for Sustainability (EfS) is crucial for changes in environmental behaviour (EB), and little is known about the EB of primary school teachers tasked with teaching EfS. This study sought to better understand the EB of pre-service primary science teachers. EB was qualitatively evaluated, characterising teachers' personal environmental activism and commitment to implement EfS among pupils and their families. Data was collected via two open questionnaires based on the Johari Window (JW) and the Authentic Inner Compass (AIC) models. Both questionnaires referred to EB twice: after exposure to the JW model and via statements from the AIC model, and again after 3 months. Findings show that the JW can reveal EB, while the AIC allowed participants to enrich their descriptions of their values, needs and commitment to EB. All participants agreed it was their duty to address EfS in class, but less than half mentioned this when describing actual behaviour. This study enables in-depth understanding of participants' EB, including their actions, barriers and concerns, which might precede planning programmes on EfS implementation. As tomorrow educators, PSTs should be the focus of such programmes, which should be part of teacher education curricula.