The hydration of a phospholipid monolayer of dipalmitoylphosphocholine (DPPC) at the air/water interface is studied for different lateral pressures which correspond the the fluidlike I and LE phases and to the solidlike S phase using the neutron reflection technique. This method enables for the first time a precise determination of the number (n(w’)) of water molecules per DPPC headgroup as a function of monolayer phase state. Furthermore, the effect of the presence of the nonionic surfactant poly(oxyethyleneglycol)n-dodecyl ether (C(12)E(4)) on the monolayer hydration was determined for the I and LE phases. For the pure DPPC monolayer, we observe a drastic dependence of n(w)’ on the phase state, with the highest hydration of n(w’) = 22 +/- 2 measured for the I phase and the lowest hydration of n(w’)= 4 +/- 1 for the solidlike S phase. In between these extremes, we obtain n(w’) = 10 +/- 1 for the fluidlike LE phase. These values correlate with those determined for the headgroup volume of DPPC at the corresponding phase states. The presence of C(12)E(4) in the monolayer at a molar lipid/surfactant ratio of 2 causes a significant increase of the monolayer hydration by 35-70% for the fluidlike I and S phases. However, the surfactant exhibits a significantly less pronounced dependence of its hydration on the lateral pressure as compared to DPPC. A comparison of our pure DPPC monolayer hydration data with published values for DPPC bilayers reveals striking differences with respect to n(w’) values for apparently similar phase states in monolayers and bilayers. We conclude that an identification of the corresponding monolayer and bilayer phases solely on the basis of headgroup hydration is not applicable.