Squarylium, or squaraine, dyes are derived from 1,2-dihydroxycyclobuten-3,4-dione, otherwise known as squaric acid. They are of two principal types: the 1,2-bis-donor substituted derivatives, and the 1,3-bis-donor substituted derivatives. The former is essentially merocyanines and have no distinctive properties, whereas the latter represent a unique type of chromophore, which is neither a merocyanines nor cyanine, and has exceptional light absorption characteristics. In this respect, types can be classified along with the H-chromophores as special cross-conjugated electron systems. They have many functional applications based on their special properties. Thus is was the objective of this research project to synthesise a range of 1,3-squarylium dyes with electron donor end groups based on heterocyclic residues, and to investigate their light absorption and fluorescence properties. During this study, a various pH-induced colour change processes, and the properties of appropriate examples in particular were examined.