Abstract <p>Expressions have been obtained to trace the time (<i>t</i>) dependence on the concentration of radicals ([R]) created by a short radiation pulse, taking into account the chain-length dependence of propagation and termination rate coefficients. These expressions have been used to test the accuracy of determining the parameters of the composite model of the termination rate coefficient by single pulse-pulsed laser polymerization-electron paramagnetic resonance (SP-PLP-EPR) method using in silico modeling. Testing has shown that to find the transition chain length <i>L</i>_<i>f</i> of the composite model, it is correct to search for the inflection point of the dependence of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\({\text{log}}\left( {\rho {\text{/}}[\text{R}] - 1} \right)\)</EquationSource> <!--PolSciA2660010Nikitin-m1--> </InlineEquation> on log(<i>t</i>), rather than determining the break point of this dependence. Also, due to the influence of the chain-length dependence of the propagation constant for short radicals, the parameters of the composite model of chain termination are determined with a significant systematic error.</p>