Ultrasonic attenuation data (frequency range 10 MHz < f < 45 MHz) of a binary isobutyric acid(COOH)/H2O mixture and a pseudo‐binary isobutyric acid(COOH)/H2O, D2O mixture of critical composition are analyzed in terms of the Ferrell‐Bhattacharjee attenuation function F(Ω). The system specific parameter ω0 necessary to calculate the temperature and frequency dependence of the universal reduced variable Ω is determined by static and dynamic light scattering experiments. It is found that F(Ω) fails to describe the ultrasonic attenuation in both systems. The data do not scale as expected theoretically and indicate a temperature and frequency dependent background attenuation caused by chemical processes. The background attenuation (α/f2)b,T reaches considerable values at frequencies f < 20 MHz and at temperatures away from the critical ((α/f2)b,T/(α/f2)T of the order of 0.1). The chemical background attenuation reflects parts of the broad spectrum of low frequency relaxation processes (frequency at the principle relaxation time f1 ≃ 2 MHz) found by Kaatze et al. (J. Chem. Phys. 93, 4955 (1989)) in a broadband ultrasonic attenuation study in a critical isobutyric acid(COOH)/H2O mixture.