Brian Thrush studied the atom and free radical reactions in the gas phase of spectroscopic methods. He made the first comprehensive examination of the absorption spectra of free radicals in homogeneous explosions using flash photolysis. He discovered the absorption spectra of several free radicals (for example, the azide, cyclopentadienyl and tropyl radicals), and he determined the ionisation potential of the tropyl radical.

He developed a new method of studying hydrogen atom reactions, and determined the rate constants of a series of nitrogen, hydrogen and oxygen atom reactions important in combustion and in the upper atmosphere. His interest in chemiluminescence yielded a series of important papers on the formation of electronically excited molecules in transfer or recombination reactions and also potential lasers.

Brian also developed a photochemical method for studying unimolecular reactions of molecules with known energies. He also studied the rotational spectra of free radicals (NH₂, PH₂, PH) using the Zeeman effect to bring them into resonance with a far infrared laser.