With atmospheric pollution causing building stone to decay at accelerated rates, conservation of historic stone monuments is becoming an increasingly important issue. Mortar joints have a marked effect on how moisture moves in a wall and, hence, on how the wall weathers. Not only does mortar bind stones together, but also a good mortar will prevent the ingress of moisture (and pollutants), enable the buildings to dry out, accommodate movement and will not act as a source of harmful chemicals capable of attacking the surrounding stone. Thus, poor mortar can have deleterious effects on the stonework. This paper, which is the result of research sponsored by the Commission of European Communities, considers a number of commonly used, and some more rarely used, pointing mortars, and assesses them with a view to using the most successful for repointing a historic stone building in Trinity College, Dublin.