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An electro-optically tunable vertically integrated As2S3 Mach-Zehnder interferometer side coupled to a Ti-diffused waveguide on LiNbO3 substrate was designed and fabricated. Asymmetric coplanar strip electrodes were placed along the As2S3 path, which show strong electro-optic tuning capability, shifting the FSR of the interferometer. The tuning of the TM polarization with 7-mm-long electrodes having a 7-μm gap was observed with VπL = 15.9 V-cm, giving a tuning rate of 0.028 nm/V at 1530 nm. To our knowledge, this is the first demonstration of electro-optic tuning of an As2S3 guided mode on LiNbO3 substrate.