It has been demonstrated theoretically that the particle size distribution of particles in a given sieve fraction of a powder may be assessed by means of short-term dissolution data. Theoretical considerations in this article show that by accounting for polydispersity in a powder sample, the cubic expression in time for amount undissolved and fraction undissolved gives rise to integrals that are essentially moments of the distribution function of one of the defining dimensions of the particle. The first and the second moments can be used to calculate the distribution parameters, (mean and standard deviation) of such a dimension of a crystalline powder. The theory is based on a model geometry, a parallelepiped, for the description of particles such as needles, plates, and prisms. The theory is substantiated by experimental data. A method for obtaining the particle size distribution parameters from the results of dissolution of three sieve fractions of oxalic acid dihydrate, and the general application of this to particle size determination is discussed. To validate the method, the distribution of lengths and breadths of oxalic acid dihydrate particles was obtained from microscopy. From actual powder dissolution data, an estimate of the mean height-to-breadth ratio of these particles belonging to a certain sieve fraction was obtained. With the knowledge of the dissolution rate constant, K, for oxalic acid dihydrate under specified hydrodynamic conditions, it was possible to evaluate the moments of the distribution function. The distribution parameters so obtained were in good agreement with the results obtained from microscopy.