We have carried out a study of the particle size distribution and aqueous dissolution rate of two commercially available qualities of orthoboric acid, labeled ''crystal'' (ABC) and ''powder'' (ABP). In a previous work, we have shown that the two commercial qualities of orthoboric acid chosen as model compound (''powder'' and ''crystal'') are related to the same crystal network in spite of their different names. However, these two qualities have very different size particle distributions, as previously determined by sieving and confirmed by the present laser light scattering study. Dissolution testing is performed under sink conditions and show that the bulk ABC quality dissolves far move rapidly that the bulk ABP quality. For each quality, dissolution rates of four sieved particle size fractions (0-90 mu m; 90-125 mu m; 125-180 mu m; 180-250 mu m) were compared. Concerning the ABC quality, comparisons were also done with three other particles size fractions: 250-355 mu m, 355-500 mu m, and 500-710 mu m, This study used the dQ/dt versus t profile. Dissolution profiles of the fractions enclosing particles with a size superior to 125 mu m are very close. On the other hand, fractions enclosing particles with a size smaller than 90 mu m present a different profile and a slower rate of dissolution.