The electronegativity in a continuous wave (CW) and pulsed mode plasmas was calculated using the measured results of both the single Langmuir probe and the retarding field analyzer. For the pulsed mode measurement, both of the measurements were performed in a time-resolved method using a boxcar sampling technique. For the conversion of the retarding field analyzer measurement results into absolute positive ion densities, argon plasma was used as a reference. The pulsed oxygen plasma was generated using the inductively coupled antenna and modulated at a repetition rate of 5 kHz and the duty ratio of 50%. The gas pressure was changed from 5 to 30 mTorr. The time evolution of the electronegativity shows that there is a pressure regime where the electron attachment reaction during the RF on-time is very active, indicating that the negative ion density reaches its maximum value during the RF on-time. Compared to the CW oxygen plasma, the electronegativity of the pulsed oxygen plasma varies within a wider range of values. (C) 2010 Elsevier B.V. All rights reserved.