In this work, the chemical resistance of six kinds of pultruded E-glass fiber reinforced isophthalic polyesters are studied. They contain 50 w% or 60 w% glass. The composites were either unfilled or filled with calcium carbonate or kaolin clay. The mechanical, physical, optical, and structural properties of the pultruded flat profiles before and after immersion in seawater, demineralized water, sodium hydroxide (5%), and sulfuric acid (10%) were analyzed over the period of one year.