Abstract: Previous work on differently substituted carbostyrils [1-3] has shown that only some especially substituted molecules of this substance class show interesting fluorescence properties. However, in contrast to the widely used oxa analogue coumarins strong fluorophors are scarce. Therefore principle strategies have been investigated, which substituent properties are important. What makes a carbostyril to be a good fluorophor? - Donor substituents in positions 6 and 7 - a trifluoromethyl group in position 4 Experimental and theoretical data have shown that the position 7 influences properties like absorption and quantum yield, whereas position 6 is connected to absorption wavelength. A strong acceptor substituent in position 4 gives a significant bathochromic shift and increases the quantum yield in most of the investigated molecules.