A technique for providing a means of maintaining molecules in an inert medium at low temperature for spectroscopic study. This method is particularly well suited for preserving reactive species in a solid, inert environment. Absorption (infrared, visible, and ultraviolet), electron-spin resonance, and laser-excitation spectroscopes can be used to examine elusive molecular fragments such as free radicals that may be postulated as important controlling intermediates for chemical transformations used in industrial reactions, high-temperature molecules that are in equilibrium with solids at very high temperatures, weak molecular complexes that may be stable at low temperatures, new reactive molecular species, and molecular ions that are produced in plasma discharges or by high-energy radiation. The matrix isolation technique enables spectroscopic data to be obtained for reactive molecular fragments, many of which cannot be studied in the gas phase.