Starting pre-school is a major stepping stone for children's peer relations. Yet, some children spend their recess time alone, albeit in the presence of playful peers. These solitary behaviours have been noted in the literature as an alarm signal for a maladaptive social development. In this study, we identified four kinds of non-social behaviours engaged at recess (reticent, solitary-pretend, -functional, and -passive); and we examined the extent to which these different behaviours were related to social solitude at the playground six months later. Therefore, 97 children (aged 4-6 years old) were observed at the playground and their social-emotional skills were tested. Solitude was assessed through an innovative measuring method, based on Radio Frequency Identification Devices. The results demonstrated that solitary-pretend play in girls was related to an increase in solitary behaviours later on. Nevertheless, children who engaged in non-social behaviours showed a general lack of emotional skills, which may explain their initial withdrawal. (author abstract)