Organic residues preserved on the outer surfaces of archaeological pottery are commonly considered to be soot and, not being subject to reservoir effects, as more reliable for radiocarbon (14C) dating compared to food crusts from the inner surface. However, unlike food crusts, outer surface residues are never analyzed prior to 14C dating. This study confronts 14C dates on inner and outer surface residues preserved on prehistoric pottery from Bazel Sluis (Belgium) with the results of stable isotope analysis and thermally assisted hydrolysis and methylation pyrolysis-gas chromatography-mass spectrometry (THM-GC-MS). These analyses clearly show that food residue is also present on the outer pottery surface, causing a possible reservoir effect on 14C dates. At Bazel, 14C dates on both the inner and outer surface residues are too old compared to dates obtained on associated animal bone. In addition, the outer surface residues systematically date younger than the inner food crusts, a discrepancy that is also known from other archaeological sites. It is suggested that these age differences are due to the mixed presence of soot and food residue on the exterior vessel wall as opposed to more homogeneous food crusts on the internal surface.