Absence of corrosion on iron nails of Roman origin from Hungate, York, found to be coated with mixed ferrous and ferric phosphates, has been attributed to the presence of tannins in surrounding soil acting as inhibitors to the activity of the sulphate-reducing bacteria in anaerobic conditions. This explanation is inadequate because an insufficiently high concentration of tannin was present. Well-preserved iron nails from St. Neots, Huntingdonshire, of early 16th century origin, were found in an aggressive salty gravel containing sulphide and active sulphate-reducing bacteria and were coated with 1/2 mm. of coherent vivianite (ferrous phosphate). A comparison has been made between the electrochemical polarization curves for these nails in anaerobic microbiological medium and for bare mild steel in similar conditions. Results can be interpreted as a restriction of the passage of ferrous ions from the metal into solution, through the coating, far in excess of that imposed by an artificially prepared phosphate coating. -- AATA