A Thomson scattering diagnostic, using an intensified gated charged coupled device detector and a high-repetition rate yttrium aluminum garnet laser, was utilized to measure electron parameters in a radio-frequency discharge. Both inductively and capacitively coupled plasmas in argon and argon-oxygen mixtures were studied, with electron densities as low as similar to 5x10(9) cm(-3). Different modes of detection (direct accumulation and photon counting) have been compared. The photon counting technique has been used to provide better accuracy at low light level (i.e., low electron densities). The results are compared with Langmuir probe measurements performed under the same plasma conditions, and good agreement is found between the two diagnostics.