The order/disorder transition of the phospholipid 1,2-dihexadecyl-sn-glycero-3-phosphocholine and its derivatives was studied by X-ray reflection and thermodynamic measurements. We find that increasing the length of an ethylene oxide spacer (zero to three EO groups) between the phosphate group and the hydrophobic chains (i) induces a slight shift of the main transition, (ii) reduces the influence of the head groups on the chain lattice, and (iii) increases the head group length and promotes its hydration. For DH(EO)(3)PC in the ordered phase we find that the small phosphate groups (which are the main source of contrast in X-ray reflectivity) are homogeneously distributed within the head group region. In the fluid phase the phosphate groups are preferentially oriented toward the air/water interface.