This study explores preschool teachers' considerations when organising children into subgroups in Swedish preschools. Grounded on interactionist perspectives and Bronfenbrenner's ecological systems theory, the study hypothesises that preschool teachers' practice of organising children into subgroups is embedded in and shaped through dynamic interactions with all actors and levels of the entire preschool system. Based on this theoretical framework, a hypothetical model has been specified that brings together variables related to children's characteristics, preschool teachers' interpersonal relationships and working conditions as well as preschools' organisational aspects. The sample consists of 698 preschool teachers from different preschools in 46 municipalities in Sweden who completed an online questionnaire. By applying Confirmatory Factor Analysis technique in evaluating the hypothesised model, the study's objective was to identify factors that determine preschool teachers' decisions when organising children into subgroups. The analysis identified a three-factor model where two residual but distinct factors, namely an "interrelational" and a "structural" factor, were nested under one general factor. The results suggest that organising children into subgroups in preschool is partly a collectively negotiated practice and partly closely connected to the complexities of preschool classrooms' actual conditions.