The ultrasonic absorption in aqueous suspensions (essentially at low concentrations) has been investigated by the pulse interferometric technique. Epstein's theory on this effect is briefly reviewed with the general expressions as well as those of limiting cases for calculating the sound attenuation coefficients derived and discussed corresponding to the actual experimental conditions.Measurements were conducted in aqueous suspensions of the pollen of pinaceae in the frequency range from 3 to 20 Mc at temperatures 20-80℃ for different concentrations. To reduce the measuring errors to a minimum, methods employing both single and multiple pulses, and comparing the pulse height in two troughs one with and the other without the hydrosols, were alternately adopted. Theoretical values based on rigorous formulae and calculated by aid of the electronic digital computer were compared with the experimental ones.Measuring data and corresponding theoretical interpretations based on approximate formulae from some other authors for aqueous suspensions of lycopodium were critically examined. It was pointed out that some of their agreement was fortuitous. The order of magnitude of absorption due to viscosity and scattering under different experimental conditions for the two suspensions was discussed in some length.