This work presents an add-on result of cross-linking using photoacoustic spectroscopy (PAS) phases in a multivariable process using the phase-resolved photoacoustic (PRPA) method. The method is tested to separate contributions from groupings -OH, -CH(2)-, -CH(3), and Si-OH overtones in the range from 700 to 2600 nm. Samples of the copolymers ethylene vinyltrimethoxysilane (EVS) and grafted vinyltrimethoxysilane (VTS) on low-density polyethylene (LDPE) were prepared having concentrations (C) of 3, 5, and 7% of catalyst and temperatures (T) of 70, 80, and 90 degrees C. By considering the condensation reaction, the reduction of -OH groups is monitored. The PRPA analysis was carried out using the surface response methodology and results indicated the optimum point for both answers (signal and phase-resolved) and for both factors of interest (C, T). For the copolymers, it was found that C approximately (5.00 +/- 1.14)% and T approximately (82.4 +/- 4.3) degrees C, while for grafted LDPE PE(g) it was found that C approximately (4.92 +/- 0.85)% and T approximately (80.8 +/- 2.5) degrees C. These results agreed with the chemical analysis for gel content, which showed as the optimum point 5% of catalyst and temperature of 80 degrees C. Further, it is an advantage that the photoacoustic method allows one to per- form a nondestructive analysis.