Abstract—Recent studies of the effects of molecular structure and reaction environment on the mechanism of primary photochemical processes involving transfer and conversion of electronic energy in relatively ‘simple’ organic molecules are presented and discussed. A quantitative i.r. spectroscopic method for studying intramolecular and intermolecular photoprocesses of u.v. irradiated substrates dispersed in solid alkali halide matrices at room temperature is described. Current data for the substratesortho‐nitrobenzaldehyde, anthracene and benzophenonebenzhydrol are presented.A series of ‘model’ ketones containing cyclopropyl groups have been synthesized and while their absorption spectra are similar, the efficiency of vapor‐phase photodissociation into radicals is shown to be strongly dependent on molecular structure.Butyrophenone and a series of ring substituted derivatives have been photolyzed in the liquid phase using the quantum yield of the photoelimination of ethylene (Type II split) as a “probe” to determine the effect of substituents on the internal H atom abstracting power of the excited carbonyl chromophore. Φc2H4is very sensitive to ring substitution, dropping from 0.24 in butyrophenone to 0.20, 0.058 and 0.00 in thep‐CH3,p‐OCH3andp‐NH2derivatives respectively, and to 0.00 in bothorthoandparahydroxy derivatives. This effect is correlated with their absorption spectra in terms of the lowest states of these alkyl aryl ketones being3(π, π*) rather than3(π, π*) in character.While several ‘classic’ photochemical reactions, unimolecular and bimolecular, proceed efficiently in solid KBr matrices giving the same product as in liquid systems, the ‘model’ cyclopropyl compounds and the alkyl aryl ketones did not undergo their usual intramolecular processes. Implications of this molecula