This paper describes a multiple case study on open inquiry-based learning in primary schools. During open inquiry, teachers often experience difficulties in balancing support and transferring responsibility to pupils' own learning. To facilitate teachers in guiding open inquiry, we developed hard and soft scaffolds. The hard scaffolds consisted of documents with explanations and/or exercises regarding difficult parts of the inquiry process. The soft scaffolds included explicit references to and additional explanations of the hard scaffolds. We investigated how teacher implementation of these scaffolds contributed to pupils' self-directed learning during open inquiry. Four classes of pupils, aged 10-11, were observed while they conducted an inquiry lesson module of about 10 lessons in their classrooms. Data were acquired via classroom observations, audio recordings, and interviews with teachers and pupils. The results show that after the introduction of the hard scaffolds by the teacher, pupils were able and willing to apply them to their investigations. Combining hard scaffolds with additional soft scaffolding promoted pupils' scientific understanding and contributed to a shared guidance of the inquiry process by the teacher and her pupils. Our results imply that the effective use of scaffolds is an important element to be included in teacher professionalisation.