The durability of cement-lime rendering mortars in marine environments (that is close to the water line) is poor due to salt crystallization, sulphate attack and moisture movement stresses. A study was carried out to establish preferred mix proportions for a marine environment and to establish the relative importance of deterioration mechanisms. Two year exposure and accelerated ageing laboratory tests were carried out. Mortar resistance was found to increase with its cement content. A 1:3 cement sand mortar is recommended. Deterioration was due mostly to salt crystallization. Moisture movement contributed little to the process and chemical attack was almost insignificant. Photographs, tables, 5 references.