By highly resolved infrared absorption spectra the dependence of aggregation of NO in neon on concentration, annealing, and deposition temperature is studied in recording the intensities of monomers in two sites (1874.54 and 1877.56 cm(-1)), of cis-(NO)(2) dimers in the symmetrical (around 1866 cm(-1)) and antisymmetrical (around 1780 cm(-1)) mode, of a special dimer around 1858 cm(-1) and a series of monomer side bands shifted by about 0.3, 0.6, and 1.8 cm(-1) due to coupling of molecules at different lattice sites. The dimer bands also exhibit a fine structure and a broad background caused by larger aggregates. The almost statistical size distribution at low concentration and condensation temperature changes to a preferential aggregation at higher concentration (>2x10(-3)) and condensation temperature (greater than or equal to 7 K) and the irreversible aggregation by diffusion at elevated temperatures is followed on a time scale of hours. A reversible conversion of special dimers at 1778.67 and 1865.48 cm(-1) to a dimer at 1857.93 cm(-1) is accelerated by lowering the temperature and attributed to a martensitic hcp to fee phase transition.