In the present work, the composition of volatile oil from leaves of Eucalyptus dunnii was studied using comprehensive two-dimensional gas chromatography (GC x GC) techniques. Structurally related compounds were found to elute mainly in specific regions of the two-dimensional space, showing orderly distribution with chemical class. Mass spectra of essential oil components were obtained from two different mass spectrometry detection methods: quadrupole (qMS) and time-of-flight (TOFMS), using the same GC x GC system under the same chromatographic conditions. Higher values of Similarity (average S of 914 with TOFMS compared to 880 with qMS) and Reverse (average R of 944 with TOFMS compared to 881 with qMS) we I re obtained with GC x GC/TOFMS showing its superior performance, which was most likely due to better sensitivity and resolution arising from the TOWS system, and lack of spectral bias. Also, the number of compounds found in E. dunnii essential oil was 15 higher when TOFMS was used. Most of these are lower abundance components or exhibit low quality mass spectra; this supports the improved sensitivity obtained with TOFMS. A linear relationship (r(2) = 0.998) between experimental retention indices (LTPRI) of 30 standard compounds obtained with GC x GC/TOFMS and GC with flame ionization detection literature retention indices is reported as an aid for compound identification.