In a combined acoustic-gravitational field (CAG), a particle is levitated at a position that depends on its acoustic properties such as the density and compressibility but is independent of its size. Thus, the density and compressibility of the particle can be evaluated by measuring the levitation coordinate in the CAG field. We have developed several methods for highly sensitive detection and reaction evaluation based on the levitation coordinate measurements of a single particle. The acoustic property of a particle can be modified by reactions occurring on the surface or inside the particle. For example, the density change of a microparticle can be induced by the binding of gold nanoparticles (AuNP) through a reaction. Proteins, coenzymes, DNA, and RNA can be detected at a zepto mole level by appropriately designing the reaction-mediated microparticle-AuNP binding. The dynamic evaluation of the ion-exchange reaction is also feasible by monitoring the temporal change in the levitation coordinate of a resin bead because an ion exchange reaction modifies the density and compressibility of the ion-exchange resin. In this account, we discuss the designs of the scheme and results for highly sensitive detection and the dynamic evaluation of ion-exchange reactions, mainly focusing on the results of our work.