A new method for the determination of the surface pK(a) of molecules adsorbed at the air-water interface is reported. This method is based on the combination of neutron reflection and surface tension measurements. The surface excess of the ionizable species is determined directly by neutron reflection, and the Gibbs adsorption isotherm is applied to the surface tension measurements to give the sum of the surface excesses of ionizable species and adsorbed counterions. From these two results, the fractional charge on the ionizable species adsorbed at the interface is obtained, which is directly related to the interfacial pK,. We have applied the method to a diblock copolymer of methyl methacrylate and (dimethylamino)ethyl methacrylate adsorbed at the air-water interface. The surface pK(a) of the polyelectrolyte part of the polymer was found to be 6.7 +/- 0.2 to be compared with a bulk value of 7.3 +/- 0.1.