THE INFLUENCE OF CHEMICAL SURFACE CHARACTERISTICS OF CHILEAN NATURAL AND MODIFIED ZEOLITES ON CHILEAN OAK CATALYTIC PYROLYSIS WAS INVESTIGATED IN THIS STUDY. CHILEAN ZEOLITE SAMPLES WERE CHARACTERISED BY NITROGEN ABSORPTION AT 77 K, X-RAY POWDER DIFFRACTION (XRD), AND X-RAY FLUORESCENCE (XRF). THE NATURE AND STRENGTH OF ZEOLITE ACID SITES WERE STUDIED BY DIFFUSE REFLECTANCE INFRARED FOURIER TRANSFORM (DRIFT), USING PYRIDINE AS A PROBE MOLECULE. EXPERIMENTAL PYROLYSIS WAS CONDUCTED IN A QUARTZ CYLINDRICAL REACTOR AND BIO-OILS WERE OBTAINED BY CONDENSATION OF VAPOURS IN A CLOSED CONTAINER. CHEMICAL SPECIES IN BIO-OIL SAMPLES WERE IDENTIFIED BY A GAS CHROMATOGRAPHY/MASS SPECTROPHOTOMETRY (GC/MS) ANALYTICAL PROCEDURE. RESULTS INDICATE THAT AFTER THE IONIC EXCHANGE TREATMENT, AN INCREASE OF THE BRØNSTED ACID SITE DENSITY AND STRENGTH WAS OBSERVED IN AMMONIUM-MODIFIED ZEOLITES. BRØNSTED ACIDS SITES WERE ASSOCIATED WITH AN INCREMENT OF THE COMPOSITION OF KETONES, ALDEHYDES, AND HYDROCARBONS AND TO A DECREASE IN THE COMPOSITION OF THE FOLLOWING FAMILIES (ESTERS; ETHERS; AND ACIDS) IN OBTAINED BIO-OIL SAMPLES. THE BRØNSTED ACID SITES ON AMMONIUM-MODIFIED ZEOLITE SAMPLES ARE RESPONSIBLE FOR THE UPGRADED BIO-OIL AND VALUE-ADDED CHEMICALS, OBTAINED IN THIS RESEARCH. BIO-OIL CHEMICAL COMPOSITION WAS MODIFIED WHEN THE PYROLYSIS-DERIVED COMPOUNDS WERE UPGRADED OVER A 2NHZ ZEOLITE SAMPLE, LEADING TO A LOWER QUANTITY OF OXYGENATED COMPOUNDS AND A HIGHER COMPOSITION OF VALUE-ADDED CHEMICALS.