Ion exchange between H+ and Eu3+ and/or Tb3+ was studied in the material modified by in situ sorption and thermal polymerization of acrylic acid in low-density polyethylene (LDPE-PAA) and in the composite system LDPE-Fe2O3-PAA. Fluorescence spectroscopy showed evidence of Eu3+ and/or Tb3+ ion exchanges in these materials. The matrix LDPE-PAA after Eu(III) ion exchange presented luminescence (excitation 265 nm). This was explained by an energy-transfer process from the matrix LDPE-PAA to Eu3+ ions. The LDPE-PAA matrix after simultaneous Eu3+/Tb3+ ion exchange exhibited Eu3+ and Tb3+ ion luminescence (excitation 265 nm), confirming an energy-transfer process from LDPE-PAA to Eu3+ ions in LDPE-PAA-Eu3+-Tb3+ matrix. Fe2O3 in LDPE-Fe2O3-PAA quenched the matrix for excitation at 265 nm and no emission at the region 400 nm was observed. The luminescence of Tb3+ ions in the matrix LDPE-Fe2O3-PAA-Tb3+ (excitation 265 nm) was partially quenched by Fe2O3. However, a weak emission of Eu3+ ions was observed (excitation 265 nm) in the matrix LDPE-Fe2O3-PAA after simultaneous Eu3+ and Tb3+ ion exchanges, suggesting an energy transfer from Tb3+ to Eu3+ ions.