The effect of prolonged (up to 960 hrs.) immersion in water on the dynamic modulus of elasticity of glass-reinforced plastics was studied on four commercial plastics of the Textolite type and on 6 experimental materials based on alkaline and alkali-free glass fiber webs. All plastics contained phenolic resins as binders. Measurement of the dynamic modulus of elasticity were made by the standard technique at a stretching stress rate of 80 kg. per min. and a bending stress rate of 20 kg. per min. The bending load did not exceed 25per thousand of the yield value measured under static conditions. The highest rate of elasticity reduction was observed during the first 240 hrs. of immersion. For all plastics, the elasticity modulus under bending stress (E bending) decreased more than the elasticity modulus under stretch (E stretch). After 960 hrs. immersion, the E stretch and E bending of the high-strength commercial constructional plastics were reduced by 2-13 and 3. 6-45per thousand, respectively. The corresponding figures for the experimental plastics were 22-34 and 28-41per thousand. The water absorption of the commercial plastics was 0.8-2. 15per thousand, that of experimental plastics 2.2-12.5per thousand. Comparison with the previously reported reduction of bending strength and elongation at break of glass-reinforced plastics when exposed to the action of water, shows that there is a relation between losses of mechanical strength and elasticity. Further study of this relation will make it possible to evaluate the mechanical strength losses by periodic non-destructive determination of the modulus of elasticity.