Ultrasonic relaxation spectra in the frequency range 0.08-400 MHz of the ions Ca2+ or Ba2+, interacting with the pentoses D-(-)-ribose, D-(-)-lyxose, and D-(-)-arabinose at concentrations of 0.5 M and molar ratio (pentose/metal(2+)) = 1 at 25 degreesC, are reported. For Ca2+ or Ba2+ with D-(-)-ribose ultrasonic relaxation spectra at various concentrations up to c = 0.7 mol/dm(3) at 25 degreesC are also reported. The ultrasonic spectra can be interpreted by the sum of three Debye relaxation processes. The one at the lowest relaxation frequency (similar to0.07-0.25 MHz) is ascribed to the 4C(1) reversible arrow 1C(4). isomerization of the carbohydrates. The relaxation process at intermediate frequency (similar to10-12 MHz for Ca2+ or 50-60 MHz for Ba2+) is interpreted as due to the partial desolvation of the cations by the carbohydrate. The "fast" relaxation process at 120-160 MHz is attributed to the diffusion-controlled approach and initial desolvation of the cation and carbohydrate. The two faster processes are interpreted according to the multistep Eigen-Tamm complexation mechanism: M2+ + carbohydrate reversible arrow (k(1)/k(-1)) M2+...carbohydrate reversible arrow (k(2)/k(-2)) M2+ carbohydrate.