In this work, termination and propagation kinetics of thiol-ene photopolymerizations are investigated via unsteady-state analysis, and termination is shown to occur though bimolecular radical-radical recombinations. The termination rate is too rapid to be resolved by standard differential scanning calorimetry (DSC) or Fourier transform infrared spectroscopy ( FTIR) unsteady-state techniques. However, a modified rotating sector technique is demonstrated to be a viable technique for the quantification of the average radical lifetimes in thiol-ene systems. The application of the rotating sector technique to thiol-ene polymerizations requires extensive theoretical developments and analysis that are also presented here. Kinetic parameters in thiol-ene systems are determined by utilizing the experimental knowledge of average radical lifetimes and analytical expressions for steady-state and unsteady-state polymerizations. Knowledge of individual rate parameters in binary thiol-ene systems, rather than ratios of rate parameters, is essential for the prediction of polymerization kinetics in complex thiol-ene systems.