The reaction rates for the thermal cycloadditions of substituted thiobenzophenones with phenylallene in CH3CN at 40 °C were determined and analysed by linear free energy correlations. Using a Hammett-type relationship, the best correlations were obtained with the σ–-parameter (ρ–= 1.09 ± 0.06) indicating that, in the transition state, mesomeric interaction occurs between –M substituents and an electron-releasing reaction centre. The small ρ–-value is in agreement with a 1,4-diradical intermediate in a (π2s+π2s+π2s)-cycloaddition.Separate correlation of electron-donating and electron-withdrawing substituents gives rise to two lines with different slopes, each with a much better correlation coefficient and ψ. This observation indicates that the thione system is relatively more senstive to electron-donating substituents than to electron-withdrawing substituents i.e. a π-repulsive saturation interaction occurs with the π-acceptor thiocarbonyl function. Additional analysis with Charton's DSP-equation (ρR/ρ1= 1.82) shows that the contribution of resonance effect is 69% and the contribution of inductive effect is 31%. Bromilow's DSP–non-linear resonance effect equation yields a value of +0.38 ± 0.01 for the electron demand parameter', indicating the enhanced responance interaction of electron-acceptor substituents with the thione system in the transition state. An inverse secondary isotope effect on the rate constant of disapperance of thione during the reaction with PhCHCCH2, PhCDCCH2, and PhCDCCD2 was observed; k2(Hα)/k2(Dα)= 0.91 ± 0.02 and k2(2Hγ)/k2(2Dγ)= 0.75 ± 0.01. These values are less than unity and consistent with attack at C-β and indicative for a change in hybridization at C-α and C-γ. The observed isotope effect when using D3CO-CHCCH2 instead of H3CO-CHCCH2, k2(H)/k2(D)= 1.20 ± 0.02 represents a rotational isotope effect arising from an increase in mass on substitution of hydrogen by deuterium. Experiments with an optically active allene (+)-PhC(H)CCHBut yielded products which were optically inactive, which points to a non-chiral intermediate i.e. the allylic diradical. We observed also a low solvent effect which is in agreement with the reaction mechanism i.e. a 1,4-diradical-mediated (π2s+π2s+π2s)-cycloaddition.