Mortar prisms made from three Portland cement types, were exposed at a city centre site (London) and at a suburban site (Horsted). At each site, a set of prisms was exposed either open to the rain or sheltered from it. The chemical analysis of the cements and of the collected rainwater during the experimental period are reported, together with estimates of dry deposited pollutants. The effects of the exposure were assessed by various physical and physicochemical tests of the surface and of the bulk properties of the exposed prisms. The weight increase, the most sensitive indicator of weathering effects, was greater for the sheltered prisms than for prisms open to the rain; the latter, however, showed more visual weathering. Increase in the porosity of the mortars resulted in increased mass gain. The higher acidity of the rain at the Horsted site had an insignificant effect on the percentage mass increase. The effect of this higher acidity was reflected, though inconclusively, by the lower bulk porosity of prisms made from Sulphate Resisting Portland Cement (SRPC) exposed at the Horsted site compared with the set similarly exposed at the London site. The effect of the cement type on the properties of the tested prisms was clearest in the decrease in the ultrasonic pulse velocity (UPV) of the exposed prisms made from Portland Blast Furnace Cement (PBFC) compared with the increase in UPV in prisms made from the other two cements. It seems that the changes in the properties of the prisms exposed to the atmosphere were associated both with the products of the reaction of atmospheric acids with the cement components and the products of continued hydration beyond the 28-day curing period.