The [4π + 4π] photodimerization process of the 9-substituted anthracene derivative crystalline 9-methylanthracene (9-MA) was investigated using time-resolved X-ray powder diffraction. The study was carried out in-situ using a CCD area detector. Sequential and parametric Rietveld refinement was applied for quantitative phase analysis. The results of traditional sequential Rietveld refinement showed that the evolution of the dimerization process can be described using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) model. The parameters of the JMAK equation were obtained successfully by parametric Rietveld refinement and suggest that the reaction follows heterogeneous nucleation and one-dimensional growth with a decreasing nucleation rate.