The sonoluminescence (SL) spectra of OH(A2Σ+) excited state produced during the sonolysis of water sparged with argon were measured and analyzed at various ultrasonic frequencies (20, 204, 362, 609, and 1057 kHz) in order to determine the intrabubble conditions created by multibubble cavitation. The relative populations of the OH(A2Σ+) v′ = 1–4 vibrational states as well as the vibronic temperatures (Tv, Te) have been calculated after deconvolution of the SL spectra. The results of this study provide evidence for nonequilibrium plasma formation during sonolysis of water in the presence of argon. At low ultrasonic frequency (20 kHz), a weakly excited plasma with Brau vibrational distribution is formed (Te ∼ 0.7 eV and Tv ∼ 5000 K). By contrast, at high-frequency ultrasound, the plasma inside the collapsing bubbles exhibits Treanor behavior typical for strong vibrational excitation. The Te and Tv values increase with ultrasonic frequency, reaching Te ∼ 1 eV and Tv ∼ 9800 K at 1057 kHz.