Nanometer-scale chemical imaging of epitaxially grown gallium nitride (GaN) and indium nitride (InN) islands is performed using scattering-type apertureless near-field scanning optical microscopy (ANSOM). The scattering of 633 nm laser radiation is modulated by an oscillating metallic probe, and the scattered radiation is detected by homodyne amplification, followed by high-harmonic demodulation, yielding optical near-field scattering maps with a spatial resolution better than 30 nm. The image contrast between InN and GaN, and the tip-sample distance dependence, can be qualitatively explained by a simple dipole-coupling model. The ANSOM images of InN and GaN also show structures that are absent in the topographic counterpart, and these substructures are explained by the variations of the local dielectric environment of InN and GaN.