A durability study was performed on a pultruded glass fiber reinforced polymer (GFRP) compression element of a hybrid GFRP/steel joint for concrete structures. GFRP elements were immersed in alkaline pore water solutions of different temperatures during 18 months. Moisture uptake occurred very quickly, within a few days, mainly through a wicking effect along the fiber/matrix interfaces and matrix cracks. The loss of matrix stiffness due to swelling led to a first rapid and significant drop in element compression strength, because of the loss of matrix resistance against buckling of the compressed fibers. In the second phase, strength reduction due to chemical glass and matrix degradation occurred at a much slower rate. It was found that the Arrhenius rate law could predict the element strength decrease. Due to the less harsh environment in practice, the strength and stiffness decrease was found to be acceptable, thereby making it possible to assure structural safety and serviceability of the hybrid GFRP/steel joint after 70 years of service. © 2007 ASCE. (26 refs.)