ABSTRACT Results are presented of studies of astronomical observing conditions, including polar startrail observations and meteorological data, at two sites in the Canary Islands. The merits of these two locations, Izaiia, on the island of Tenenfe, and Fuente Nueva, on the island of La Palma, as sites for astronomical observatories are discussed. Seeing conditions at Izaiia, while good, were not excellent; at Fuente Nueva, the seeing observed during the peried of observations was as good as or better than that so far observed at any other site where the polar star-trail method of testing has been used. This difference is interpreted in terms of the topography of the islands. The best astronomical observing conditions probably occur on islands, in cold oceans or regions with maritime tropical stable air masses, having the shape of a conical peak rising out of the sea, where the peak is of sufficient height that its top rises above the top of the inversion layer into the region where the aifflow is laminar as a result of its passage over the ocean. Other islands whose location and topography meets these conditions are discussed. Key words: site testing - astronomical seeing - Tenerffe - La Palma