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Traditional Balsamic Vinegar of Modena (TBVM) is a product of Modena district (Emilia Romagna, Italy), well known, and largely appreciated worldwide. During the ageing of homemade TBVM, the concentration of solute produces an increase in density (mainly owing to the evaporation of water), the balance between sweet and acidic tastes, and the formation of dark brown colour. This paper considers 20 samples of homemade TBVM supplied by private owners of Modena district. Since about 20 years, undesirable effects of crystallization of homemade TBVM are becoming common during the period of settling in the wooden barrels, and after the ageing period in the bottles. Although the process to control crystallization of wine is optimized, in the case of homemade TBVM a protocol to control the crystallization during ageing has not been clearly defined yet. X-ray powder diffraction (XRPD), a common technique used in mineralogy and inorganic chemistry, provided an unambiguous identification of crystal by-products in TBVM. XRPD patterns of ABTM_extra 1, ABTM_extra 2, ABTM_extra 3, and ABTM_trad 1 show the presence of glucose crystals in the bottom of the bottles. XRDP analyses suggest that the glucose deposit can primarily related to the high concentration of the starting cooked must, which determines the imbalance of glucose/fructose ratio and the growth of glucose crystals during ageing. Refractive and titration analyses in ABTM_extra 1, ABTM_extra 2, ABTM_extra 3, and ABTM_trad 1 showed that sugar content and total acidity are >72°Brix and ≤6 g/L, respectively. These results can be ascribed to the high concentration of starting cooked grape must, evaporation of water during ageing, and ageing time. This paper describes the relationships between the sugar content, total acidity (volatile and non-volatile acidity), and crystal by-products during TBVM ageing to suggest the best practice for controlling or eliminating the crystallization. Moreover, this paper shows that the R ratio (°Brix/total acidity) is a good reference parameter to control crystallization in homemade TBVM during ageing. [ABSTRACT FROM AUTHOR]