Abstract The tunnel effect theory is applied to a study of the efficiency of hydrogen photoabstractions by excited states of carbonyl compounds. Efficiency depends on the energy of the crossing point ω of the potential energy curves of S 0 and of the radical intermediate state I and consequently depends on the CH bond strengths, the oscillator displacements, the nature of the electronic states, the ionization energy of the substrates, the affinity of the ketones and the solvent polarity. The theory shows that the dependence of the efficiency on the molecular structure provides a good criterium to distinguish between the mechanisms of tunnelling and thermal activation. Experimental data support the view that hydrogen photoabstractions occur via a tunnelling mechanism and not via a thermal activation process. Qualitative rules of structure—efficiency relationships are presented.