CONFLICTING reports have been published in recent years on the reliability of radiocarbon dating of mortars. Delibrias and Labeyrie1 found accurate dating to be possible, but Stuiver and Smith2 found incorrect age assessments. The observed discrepancies were attributed to the dilution of 14C activity by ancient carbon which may have been a residue of the limestone from which the “CaO” was initially prepared. In addition a minor dilution effect might result if calcareous sands were used in the manufacture of the original aggregate. We have investigated further the validity of the dating technique, using a number of British mortars of accurately known age and origin provided by the UK Ministry of Public Building and Works, Ancient Monuments Branch. Radiocarbon analyses of these samples were performed by methane gas counting following acid hydrolysis of the samples and conversion of the product carbon dioxide to methane by catalytic hydrogenation3.