This communication describes a new type of alloyed Cd–In–S quantum dots (CdIS QDs) with ultra small particle size and broadly tunable fluorescence emission from 450 to 700 nm. The band gap of CdIS QDs was mainly controlled by their composition rather than their particle size. The CdIS–ZnS core–shell nanocrystals exhibited significantly improved optical properties and chemical stabilities, with the PL quantum yield (QY) of up to 60%.