Ultrasonic attenuation spectra (30 kHz less than or equal to nu less than or equal to 400 MHz) of the isobutyric acid/water mixture of critical composition and also of the acid itself (50 kHz less than or equal to nu less than or equal to 1100 MHz) are discussed at different temperatures. Quasielastic light scattering data from photon correlation spectrometry of the critical system are evaluated to yield the amplitude D-0 of the mutual diffusion coefficient in the homogeneous phase. Using literature values for the amplitude of the fluctuation correlation length, the background and critical part of the heat capacity, and the linear coefficient in the pressure dependence of the critical temperature T-C, the sonic attenuation spectrum as predicted by the Bhattacharjee-Ferrell model has been calculated for the critical mixture at T-C. Following again this theoretical model, the contribution due to concentration fluctuations at the temperatures of measurement and also the high-frequency asymptotic background contribution has been subtracted from the experimental spectra. The resulting excess attenuation spectra of the isobutyric acid/water mixture reveal two relaxation processes, both characterized by a discrete relaxation time. These Debye-type relaxations are discussed in terms of the monomer/linear dimer and linear dimer/cyclic dimer equilibria of the carboxylic acid. The relaxation times of the mixture of critical composition exhibit slowing characteristics in the chemical reactions near the critical temperature (T - T-C < 5 K) which cannot be explained by the critical behavior of the viscosity. Rather there seems to be an intrinsic effect that slows down near the critical point.