The effect of the ratio of the electric field to the buffer gas number density (E/N) in the drift tube reactor of a proton transfer reaction-mass spectrometer (PTR-MS) on the product ion distributions of seven common biogenic unsaturated alcohols (2-methyl-3-buten-2-ol, 1-penten-3-ol, cis-3-hexen-1-ol, trans-2-hexen-1-ol, 1-octen-3-ol, 6-methyl-5-hepten-2-ol and linalool) has been investigated. At low E/N values, the dominant product ion is the dehydrated protonated alcohol. Increasing E/N results in more extensive fragmentation for all compounds. For cis-3-hexenol and 6-methyl-5-hepten-2-ol the contribution of the protonated molecule can be enhanced by reducing E/N with respect to commonly used PTR-MS E/N values (120-130 Td). Significant differences have been found between some of the isomeric species studied, opening a way for selective detection. The C10 alcohol linalool mainly results in product ions at m/z 137 and 81, which are also PTR-MS fingerprints of monoterpenes. This may complicate monoterpene quantification when linalool and monoterpenes are simultaneously present in sampled air. Furthermore the influence of the water vapour pressure in the PTR-MS inlet line on the product ion distributions has been determined. Some major fingerprint ions of the unsaturated alcohols were found to depend significantly on the water vapour pressure in the inlet line and this should be taken into account for accurate quantification of these species by PTR-MS.