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The effect of an iodine atom, when present in 5-position of the benzene nucleus of thionaphthene ring, has been studied with regard to benzaldehyde series. For this purpose four new dyes have been synthesised by condensing the 5-iodothioindoxyl with benzaldehyde, \(p\)-nitrobenzaldehycle, \(p\)-dimethylaminobenzaldehyde, and \(p\)-diethylaminobenzaldehyde. Their dyeing and other properties have been studied. Attempt has been made to explain the difficulty in vatting from the view point of ionic mechanism. These dyes are feeble but definite sensitisers. The absorption maxima and sensitisation spectra of these dyes are reported.