The influences of curing agents and the water absorption on the microstructure have been studied by positron annihilation lifetime spectroscopy (PALS) and gravimetric measurement for epoxy resins DER331 (E51). We found that the o-Ps lifetime 3τ dramatically decreases with the immersion time from 0h to 6h, and then reaches a saturate level in higher water absorption, while the o-Ps intensity almost remains a constant for all samples. This fact indicated that the water was first filled into free volume holes at low water content. The observation of the constant of I3 suggested that the free volume concentration was not affected by the water sorption. Experimental results indicated that both chemical reaction between the positronium and chlorines and electronegative chlorines attracting positrons can reduce the positronium formation.