In the present study, we compared the anticancer potential of quercetin (3,3',4',5,7-pentahydroxyflavone, I) and its sulfonic derivatives sodium/potassium quercetin-5'-sulfonates (described as II and III) against several human carcinoma cell lines. Quercetin (I) was used as a starting compound for synthesis of II and III. In this work, a modified and more efficient method of synthesizing derivatives II and III has been described. The molecular structures of the compounds were characterized in a solution and in the solid state using H NMR, C NMR, 2D NMR, and XPS spectroscopy, respectively. The stoichiometry of these complexes was determined by elemental analysis as well as thermogravimetric and X-ray fluorescence methods. The spectral data allowed complete characterization of the investigated compounds in the solution and in the solid state and unambiguous determination of the place of substitution of the sulfonic group in the phenyl ring in the C-5' position. Our in vitro studies revealed that II and III prominently reduced the viability of the HT-29 colon cancer cell line. Additionally, we observed that sulfonic derivatives decreased proliferation of colon (HT-29, LS180), lung (A549), and breast (T47D) cancer cell lines. Moreover, we detected a lower cytotoxic effect of II and III on several normal cell lines (colon epithelial CCD 841 CoTr, mouse subcutaneous connective tissue L-929, and human skin fibroblasts HSF cell lines) than that exerted by pure quercetin. The anticancer properties were especially evident in the HT-29 colon cancer cell line, where cell cycle inhibition in the G-M phase and prominent apoptosis induced by II and III were observed. In conclusion, the sodium/potassium quercetin-5'-sulfonates prepared from quercetin showed promising anti-proliferative and pro-apoptotic activity against colon cancer cells. Therefore, we support the opinion that sodium/potassium quercetin-5'-sulfonates should be considered as promising organometallic compounds for possible clinical applications.