Pyrolysis of thin films of cellulose was performed at five pressures from 4 mbar (vacuum) to 1 bar at 500 degrees C in a modified pyroprobe captive sample reactor designed to minimize gas-phase secondary reactions. Known molecule products were identified and quantified by gas chromatography (GC), GC/mass spectrometry (MS), and high-performance liquid chromatography, and yields were calculated. Vacuum allowed the evaporation of cellobiosan and promoted the release of heavy oligosaccharides by thermal ejection (cellotriosan, etc.), reducing in this way the residence time of the oligomeric products in the liquid intermediate and, consequently, their secondary reactions. The char yield increased from 3.6 to 8.3% in the range of pressures studied. The unknown heavy fractions in the oil were studied in more detail by high-resolution mass spectrometry using positive-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry. The results illustrated a dominant dehydration reaction pathway in the liquid intermediate to form distinct parent molecules from which fragmentation reactions occur. Notable reactions were decarboxylation, loss of glycoaldehyde, 5-(hydroxymethyl)furfural, and 1,4:3,6-dianhydro-a-n-glucopyranose, and gain of hydrogen, methanol, and water. Furthermore, as the pressure increased, cross-linking and polyaromatic ring formation in the liquid intermediate were enhanced. A new cellulose pyrolysis reaction scheme stressing the secondary reactions in the liquid intermediate was proposed to reflect the results of this study.