Measurements of acoustic absorption and velocity as a function of frequency and concentration in KH2PO4−K2HPO4 buffers at 4°C and pH 5-7 are reported. The dependence of the observed acoustic relaxation parameters on concentration is consistent with that to be expected from perturbation of a monomer-dimer equilibrium with an equilibrium constant [for 2H2PO 4 − ⇌(H2PO4)22−] of 0.21 M−1, a bimolecular rate constant of 5×108 M−1-sec−1 and a standard volume change of −5 cm3 mole. The equilibrium constant for H2PO 4 − + HPO42−⇌H3(PO4)23− is estimated to be 0.7 M−1.