The spatial increase of pre-breakdown ionization currents has been measured in oxygen for gas pressures in the range 93-650 torr and for 39·6 <or= E/p₀ <or= 50·3 v cm^-1 torr^-1 (where E is the electric field and p₀ the pressure corrected to 0°C). The values of the Townsend generalized secondary ionization coefficient ω/α determined from the measurements decreased rapidly as E/p₀ increased from 39·6 to about 46 v cm^-1 torr^-1 and then became relatively constant as E/p₀ increased further. Values of the apparent primary ionization coefficient α/p₀, which were also determined as part of this investigation, increased with decreasing pressure at a constant value of E/p₀ for E/p₀ < 43 v cm^-1 torr^-1; the significance of this variation of α/p₀ with p₀ in the determinations of ω/α from measurements of sparking potentials is discussed.