The series of three 4-alkoxy-2-[2-hydroxy-3-(4-aryl-1-piperazinyl)propyl]-6-methyl-1H-pyrrolo [3,4-c] pyridine-1,3(2H)-diones were synthesized. The structures of all formed compounds were identified by elemental analysis, FTIR, and 1 H NMR. The synthesized compounds were studied using differential scanning calorimetry and thermogravimetric analysis in order to determine the existence of multiple solid-state forms. Our measurements showed that the solvent, mechanical treatment, and quenching are the important factors. Two stable and one metastable modification were detected. Additionally, DFT calculations were performed.