A mathematical model is considered for the physical dissolution of a single spherical particle in a large volume of a liquid. In addition to conventional consideration, the effect of nonstationarity at the initial stage of the process and the influence of movement of the phase boundary are taken into account. The problem of finding the law of movement of the boundary is solved by fractional differentiation. For the case when the saturation concentration is much lower than the particle density, an explicit expression is derived to calculate the time of complete dissolution. It is found that the time of complete dissolution is less than that obtained using the conventional formula.