The extent of the ultraviolet-visible (UV-vis) photoirradiation effect on high-density polyethylene (HDPE) and HDPE-silicon (Si) composites is reported in terms of the addition of Si microparticles at contents of 0.1, 1, and 5 wt %. A standard accelerated UV-vis exposure was applied over 2750 h, corresponding to 22 months in Florida. Thermogravimetry, differential scanning calorimetry, and Fourier transform infrared spectroscopy were used as reliable techniques for monitoring the quality of the HDPE-Si composites. The increasing addition of Si microparticles delayed the photodegradation of the HDPE-Si composites. Because of their strong lightscattering effects, Si microparticles blocked the degradation of tertiary carbons of the HDPE backbone and reduced the apparition of vinyl groups; this prevented the structural impoverishment of HDPE-Si composites. Consequently, variations in the crystallization temperature (Tc) and melting temperature (Tm), which were indicators of photodegradation, were not modified. In general, the HDPE-Si composite formulation with 5 wt % Si microparticles was useful for protecting the material from photodegradation and, thus, should be an environmentally friendly, reliable alternative UV-vis blocker.