The impedance changes in tumor spheroids subjected to chemotherapeutic anti-cancer drugs provide real-time insights into drug efficacy. Studying tumors at the microscale requires advanced tools to detect changes at the cellular level. We developed an in-house microelectrode array (MEA) with interdigitated electrodes (IDE) to increase the electrode surface for sensitive detection and evaluated the changes upon chemotherapeutic treatments using these MEAs and appropriate analysis. Four well MEAs with six IDEs in each well were fabricated by the lithography technique. H460 lung spheroids and HCT-116 colorectal cancer spheroids (~ 200µm diameter each) were cultivated on the IDEs. The following chemotherapeutic treatments (Cisplatin [50 µM], FOLFIRI) were applied 24 hours after the spheroids placement. Impedance spectroscopy using voltage excitation and a spectrum range of 100 Hz to 1 MHz with the 2-Point technique was performed for 5 days [1]. For the analysis, a modified form of the relative impedance change equation [2], was employed to yield values ranging from -100% to 100%. The modified equation is defined as: RI change = [(Zs – Zm) / (Zs + Zm)] * 100%, where Zs and Zm are the relative impedance with spheroid and medium, respectively, and the maximum relative value is chosen. The MEA demonstrated stable impedance measurements and enabled easy detection of tumor spheroids. FOLFIRI treatment showed a small change in impedance (Fig. 1, panel B). Despite FOLFIRI having a cytotoxic effect by inhibiting DNA synthesis, the response may be limited on HCT-116 spheroids, reflecting the chemoresistant nature of these colorectal cancer cells [3] [4]. On the other hand, Cisplatin induced a significant apoptotic response, reducing impedance amplitude to around 50% (Fig. 1, panel A). These results were validated with a viability test conducted at the end of each experiment, reflecting the chemoresistance of HCT116 spheroids against FOLFIRI and the pro-apoptotic effect of Cisplatin on H460 spheroids. For control validation, H460 cells demonstrated stable impedance between 24 and 48 hours after placement, followed by a marked decline (Fig. 1, panel C), possibly due to necrotic core formation associated with their rapid proliferation. In contrast, HCT-116 cells exhibited a gradual impedance decrease, consistent with a slower growth rate (Fig. 1, panel D). In conclusion, our study demonstrates that impedance spectroscopy using MEAs with interdigitated electrodes offers a reliable method to monitor chemotherapeutic effects on tumor spheroids in real time. The noticeable impedance change observed for Cisplatin and to a smaller degree for FOLFIRI treatments revealed the capability to differentiate between drug efficacies. By analyzing relative impedance changes, we successfully quantified drug-induced cellular effects, with results aligning with viability assays. Our microelectrode-based bioimpedance platform has proven its capability to resolve the distinct proliferative signatures of lung and colorectal carcinoma cell lines, confirming its robustness for real-time, long-term evaluation of chemotherapeutic efficacy. Leveraging this sensitivity, our future studies will extend this bioelectronic approach to immunotherapeutic regimes, enabling patient-specific profiling of tumor-immune interactions for personalized oncology.