The topotactic redox reaction of K0.52Ti6Se8 with molecular iodine in acetonitrile allows the preparation of the binary titanium selenide Ti3Se4 with the hexagonal Nb3S4-type structure. The species formed during the redox reaction was identified as the complex KI.I2. The reaction of Ti3Se4 with water leads to the formation of the new ternary compound (H2O)1.5Ti6Se8. Simultaneous thermogravimetric analysis and mass spectrometry shows the emission of water over a wide temperature range and a weight loss which is in accordance with the content determined by single crystal X-ray work. The shape of the emission peak is indicative for a diffusion-limited reaction. The crystal structures of Ti3Se4 and (H2O)1.5Ti6Se8 are presented. In the water containing sample the oxygen atoms are displaced from the center of the hexagonal channels towards the TiSe6 octahedra. This displacement leads to the formation of weak Ti-O and Se-O interactions. The average O-O distances within the channels are indicative for the formation of hydrogen bonds. The intercalated water exhibits only limited influence on the crystal structure of the host material.