The structure of the lamellar phase formed in the heptaethylene glycol n-hexadecyl ether (C16E7)-water system has been studied using small-angle X-ray scattering (SAXS). The concentration dependence of the repeat distance d follows the power law d proportional to phi (hc) (-s) where phi (hc), is the volume fraction of hydrophobic layer. The exponent s obtained is unity as expected at 75 degreesC where the "L-alpha" phase extends to the lower concentration range. As the temperature decreases below about 70 degreesC, however, s decreases rapidly. Below about 55 degreesC, s becomes constant again and takes values of about 2/3. From the line shape analysis of SAXS in the "L alpha" phase, half-thickness of the hydrophobic layer delta (hc) and the thickness of the hydrophilic layer have been obtained. It has been shown that the anomalous behaviors of the exponent s cannot be explained by the variation of delta (hc) with concentration and temperature. In other words, the structure of the "L-alpha" phase varies with concentration and temperature. Assuming that the bilayer sheets have water-filled defects, we have made a "map" showing how the fraction of the defects f(w) depends on concentration and temperature. Strong correlation has been found among the f(w) value, the appearance of the broad component superimposed on the first-order reflection, and the quadrupolar splitting of (H2O)-H-2. Relations between the phase behaviors and the structures of the L-1, "L-alpha", and V-1 phases have been discussed based on these results.