Utilization of air-filled ionization chambers with 60Co-based reference calibrations in proton dosimetry requires application of water to air stopping power ratios and the mean energy required to produce an ion pair (W or w). Accepted uncertainties in current w values for protons leads to a dosimetric uncertainty of 4 per cent when ionization chambers are employed to measure absorbed dose. For this reason, proton dosimetry protocols recommend the use of calorimetry as the absorbed dose standard. We used calorimetry in conjunction with an ionization chamber with 60Co reference calibrations to deduce the proton w value in the entrance region of a 250 MeV proton beam: 34.2 +/- 0.5 eV. Application of this w value, with its 1.5 per cent uncertainty, allows determination of dose in therapeutic proton beams, with uncertainties comparable to photon and electron values.