The opposite enantiomers of pseudoephedrine were observed to react in the solid state to form the racemic compound. A novel powder X-ray diffractometric method was developed for studying the kinetics and mechanism of this reaction. The powder X-ray diffraction patterns of the enantiomers were identical, but the racemic compound exhibited a different pattern because of its different crystal structure. The powder patterns of the enantiomers and the racemic compound revealed that the 4.62 and 4.25 A lines were unique to the enantiomers and the racemic compound, respectively. Teflon holders were filled with equimolar mixtures of the two enantiomers, and the integrated intensities of the 4.62 and 4.25 Å lines were measured at 30-min intervals over a period of 8 h. Thus, the disappearance of the crystalline enantiomers and the appearance of the crystalline racemic compound were quantified simultaneously. The rate of disappearance of the crystalline enantiomers followed a diffusion-controlled reaction model originally proposed by Jander in 1927. During the kinetic experiment, the sam of the weight fractions of the enantiomers and the racemic compound progressively decreased from an initial value of unity suggesting the existence of an intermediate noncrystalline phase. The presence of the noncrystalline phase was confirmed by a steady increase in the background of the X-ray patterns. Thus, the unique ability of the X-ray method to quantify only the crystalline phases has demonstrated the existence of a non-crystalline amorphous intermediate in this solid-solid reaction.