Using eclipse laser photo-detachment in conjunction with Langmuir probing, the density of O- ions in a reactive pulsed magnetron (100 kHz, 55% duty) plasma has been determined at different times during the pulse period at a set of positions along the centre line axis of the discharge. The magnetron was operated at a fixed average power of 400W with an oxygen partial pressure of 10% of the total pressure 1.33 Pa. The results show the plasma is weakly electro-negative, with a negative ion-to-electron density ratio a up to a maximum of 0.63. During the plasma on-phase (at all chosen measurement positions) the O- density was found to reach a maximum directly after initiation of the voltage pulse decreasing weakly during the rest of the on-phase. On the transition from on-to-off phases of the pulse the negative ion density was found to fall (by 60% both close and far from the target but only 10% near the discharge centre), with the O- density remaining almost constant during the rest of the afterglow. The spatial structure of the O- density reveals a distinct peak 75 mm from the target close to but not at the position of the null in the magnetic field, falling by a factor of eight for increasing distances up to 30 mm both towards and away from the target. The highest O- density recorded at this position was 1 x 10(16) m(-3), at a time of 2.12 mu s into the pulse. From a comparison between on- and off-phase densities and using an intuitive model of the plasma, the results indicate that most negative ions are created in the bulk plasma. The density of target-borne O- ions is estimated to be about 1 x 10(15) m(-3) varying little with position, possibly forming a beam-like structure. (C) 2010 Elsevier B.V. All rights reserved.