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 1H 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.