The forces acting between two cellulose surfaces and a cellulose surface and silica have been investigated using the interferometric surface force technique and the scanning force microscopy colloidal-probe technique. A key element in this study is the preparation of very smooth cellulose surfaces using a Langmuir Blodgett technique, which facilitates a detailed characterisation of the forces acting between the surfaces as a function of their separation. The main part of the investigation is concerned with the effect of a highly charged cationic polyelectrolyte, PCMA, on the interaction between negatively charged silica and uncharged cellulose. It is found that the presence of the cationic polyelectrolyte reduces the force barrier against flocculation, but also the attractive pull-off force. The implications of these findings for the function of this polyelectrolyte as a retention aid and a wet strength additive is discussed. The ionic strength of the solution has profound effects on the interactions between cellulose and silica in the presence of PCMA. This is due to a decreased polyelectrolyte-surface affinity at higher ionic strengths.