Methods to determine Al and Si in both fuel oils and crude oils by X-ray fluorescence (XRF) spectrometry were developed. Since interference from the sample matrix was found critical in direct analysis of liquid samples, determinations were performed on samples as thin films deposited onto a solid substrate (filter paper). Quantification using the analytical curve, prepared with organic standards, was found to be adequate to determine the analytes in fuel oils by energy dispersive (ED) XRF, with limit of detection (LOD) of 1.6 mu g g(-1) (Al) and 1.2 mu g g(-1) (Si). In crude oils, Al and Si could only be successfully determined by EDXRF after ashing and fusion of samples with Li2B4O7 (following sample preparation reported in the ASTM D5184), providing samples in aqueous solutions that required matrix-matched calibration (inorganic standards in solution containing Li2B4O7 and placed onto solid substrates) for quantification, leading to LOD of 1.3 mu g g(-1) (Al) and 0.7 mu g g(-1) (Si). Direct analyzes of crude oils onto solid substrates, after only a dilution with tetrahydrofuran (THF), were only possible by wavelength dispersive (WD) XRF, which provides selectivity and sensibility enough to determine the original Al (LOD 0.5 mu g g(-1)) and Si (LOD 0.3 mu g g(-1)) in real samples using matrix matched standards made with white clay certified material dispersed in mineral oil diluted in THF.