The effect of solution ionic strength on the dissociation and unfolding of glucose oxidase (GOx) adsorbed at the air/water interface has been studied by specular neutron reflection. The structure and composition of the GOx layers was determined at pH 7 under null reflecting water (NRW) comprised of D2O and H2O with a molar ratio close to 1:11. Under this isotopic contrast, the specular reflectivity arises entirely from the adsorbed layer. At the lower ionic strength of 4.5 mM, GOx adsorption forms a uniform layer of some 30 Angstrom thick. The layer thickness tends to increase with bulk GOx concentration and eventually reaches the short axial length of the enzyme's globular monomer structure of 37 Angstrom. These results suggest that while the enzyme is dissociated into monomers it does not unfold further into the distributions characteristic of random polypeptide segments. At the higher ionic strength of 0.45 M, the thickness of the GOx layers is between 40 and 50 Angstrom when the bulk GOx concentration is below 0.4 muM. This dimension is close to the short axial length of the globular dimer of 52 Angstrom, suggesting that at the higher ionic strength the enzyme adsorbs with its long 80 Angstrom axis parallel to the surface of water. As the concentration is increased above 1 muM, a secondary layer starts to form on the solution side of the main surface layer, leading to a bilayer adsorption of some 50 Angstrom for each layer. Although the inner solution layer only contains some 5-8% of the adsorbed material, neutron reflection is sufficiently accurate to detect its structural dimension. Further neutron measurements under D2O suggest that the dissociated monomers and the dimers have different extent of immersion into the aqueous subphase. This observation is consistent with the different surface activities of the two species, which are related to the distributions of amino acid groups on their outer surfaces.