Determining the surface free energy of paper from contact angle data for various liquids is not straight forward and the results can sometimes be misleading. For insufficiently sized paper, errors may arise from rapid penetration of liquid probes into the sheet, which prevents accurate measurement of contact angles. For well-sized paper, errors may arise from factors associated with the roughness of the sheet. Despite the fact that some of the sources of error are well known, contact angle methods are still favoured for determining the surface free energy of paper. In this study, we focus on some factors associated with the topographical and chemical heterogeneity of paper (particularly sized sheets) that may cause contact angle data to lead to incorrect prediction of surface energetics. Whatman filter paper with and without calendering and AKD sizing treatment was used as a model system for studying the effect of sheet surface heterogeneity. To evaluate the sheet surface roughness at an interfibre pore level, the contact area between a water drop and the sized sheet surface was studied using confocal laser-scanning microscopy (CLSM). For well-sized sheets the actual contact area between the drop and the sheet was much smaller than the apparent contact area. The chemical heterogeneity of the sized filter paper sheets was studied using XPS and IGC. Both techniques again showed that the surfaces of well sized papers still possess acid and base polarities that are capable of interacting with liquid or vapour probes with acid and base functionality. Poor liquid–sheet contact area, however, acts as a factor that limits the number of acid–base adducts formed across the liquid–sheet interface. A possible way of correcting for the effect of sheet surface roughness on the apparent liquid–sheet contact angle using the Cassie–Baxter equation was tested.